Constant velocity universal joint

ABSTRACT

An annular support ring  32  is fitted onto the outer periphery of each trunnion  22 . This support ring  32  and the roller  34  are unitized via a plurality of needle rollers  36  to constitute a roller assembly capable of relative rotations therebetween. In longitudinal section, each trunnion  22  has an outer periphery of straight shape, parallel to the axis of the trunnion  22 . In cross section, the trunnion  22  has a generally ellipse shape with the major axis orthogonal to the axis of the joint. The inner periphery of each support ring  32  is arcuate and convex in section. This combines with the general elliptic cross sections of the trunnions  22  and the provision of predetermined clearances between the trunnions  22  and the support rings  32 , to allow the support rings  32  to move along the axial directions of the trunnions  22  as well as make tilting movements with respect to the trunnions  22.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a constant velocity universal joint foruse in power transmission devices in motor vehicles and variousindustrial machines. In particular, the invention relates to a tripodtype constant velocity universal joint.

[0003] 2. Description of the Related Art

[0004] Tripod type constant velocity universal joints are used, forexample, as an element in a power transmission device for transmittingrotational power from a car engine to wheels.

[0005] In general, a tripod type constant velocity universal joint ischiefly composed of an outer joint member and a tripod member. The outerjoint member has an inner periphery provided with threeaxially-extending track grooves. Each of the track grooves has axialroller guideways on both sides. The tripod member is provided with threeradially-projecting trunnions. A roller is rotatably arranged on each ofthe trunnions. The trunnions of the tripod member and the rollerguideways of the outer joint member engage with each other in thedirection of rotation via the rollers so that rotational torque istransmitted from a drive side to a driven side at constant velocity. Theindividual rollers rotate about the trunnions and roll on the rollerguideways as well, absorbing relative axial displacements and angulardisplacements between the outer joint member and the tripod member. Inthe meantime, also absorbed are axial displacements of the individualtrunnions to the roller guideways, the axial displacements resultingfrom phase changes in the direction of rotation when the outer jointmember and the tripod member transmit rotational torque with someoperating angle therebetween.

[0006] Some tripod type constant velocity universal joints have therollers mounted on cylindrical outer peripheries of their trunnions viaa plurality of needle rollers. When an outer joint member and a tripodmember transmit rotational torque with an operating angle, however, thetrunnions tilt to make the rollers and the respective roller guidewaysoblique to each other. This produces a slide therebetween, giving riseto a problem that resistance here hampers the smooth rolling of therollers and thereby increases induced thrust. Moreover, there is anotherproblem that the resistance between the rollers and the respectiveroller guideways increases the slide resistance to axial relativedisplacements between the outer joint member and the tripod member. Suchinduced thrust and slide resistance contribute to the production ofvibrations and noises from a car body, affecting the NVH performances ofthe motor vehicle. Typical automotive NVH phenomena associated with suchinduced thrust and slide resistance include the rolling of a moving carbody and the vibrations of a car idling with its automatic transmissionat D range, respectively. The essence of solution to the automotive NVHproblems consists in reducing the induced thrust and slide resistance inthe joint. In general, induced thrusts and slide resistances in a jointtend to depend on operating angle of the joint. This tendency leads to adesign limitation of prohibiting greater operating angles when, forexample, a constant velocity universal joint is applied to an automotivedrive shaft. Accordingly, reduction and stabilization of the inducedthrust and slide resistance are also desired for the sake of enhanceddesign flexibility of portions around the car axles.

[0007] Conventionally, to eliminate the oblique states between therollers and the roller guideways to lower the induced thrust and slideresistance, there have been proposed and put into practical use avariety of tripod type constant velocity universal joints that comprisemechanisms (roller assemblies) for allowing tilting movements of therollers with respect to the trunnions. Among the known tripod typeconstant velocity universal joints of this kind is a constitutioncomprising outer rollers to be guided by the roller guideways and innerrollers rotatably supported by the outer peripheries of the trunnionsvia a plurality of needle rollers. This constitution is then broadlydivided into the following modes a) to d).

[0008] a) The outer rollers are provided with outer peripheries ofconvex spherical shape (including both a “perfect spherical surface,”having its center of curvature on the trunnion axis, and a so-called“torus surface,” having its center of curvature off the trunnion axistoward the outer-diameter side) and inner peripheries of cylindricalshape, and the inner rollers are provided with outer peripheries ofconvex spherical shape, so that slides between the cylindrical innerperipheries of the outer rollers and the convex-spherical outerperipheries of the inner rollers permit the tilting movements of theouter rollers (Japanese Patent Publication No. Hei 3-1529, etc.).

[0009] b) The outer rollers are provided with outer peripheries ofconvex spherical shape (including both a perfect spherical surface and atorus surface) and inner peripheries shaped so as to make line contactwith outer peripheries of the inner rollers, and the inner rollers areprovided with the outer peripheries of convex spherical shape, so thatslides between the inner peripheries of the outer rollers and theconvex-spherical outer peripheries of the inner rollers permit thetilting movements of the outer rollers. Besides, the inner peripheriesof the outer rollers are shaped so that load components toward thetrunnion extremities are created at the contact positions with the outerperipheries of the inner rollers (Japanese Patent Laid-Open PublicationNo. Hei 9-14280, etc.).

[0010] c) The roller guideways are provided with flat surfaces, theouter rollers are with outer peripheries of cylindrical shape and innerperipheries of concave spherical shape, and the inner rollers are withouter peripheries of convex spherical shape, so that slides between theconcave-spherical inner peripheries of the outer rollers and theconvex-spherical outer peripheries of the inner rollers permit thetilting movements of the outer rollers (Japanese Patent ApplicationsNos. Hei 8-4073 and 8-138335).

[0011] d) In addition to the constitution c) above, the roller guidewaysand the axes of the trunnions are configured not to be parallel to eachother at an operating angle of 0° (Japanese Patent Laid-Open PublicationNo. Hei 11-13779).

[0012] Also known as a tripod type constant velocity universal joint ofthis kind is the constitution e) in which: the outer peripheries of thetrunnions are shaped into a convex spherical surface (a perfectspherical surface having the center of curvature on the trunnion axis);the rollers are mounted onto support rings via a plurality of needlerollers to constitute roller assemblies; and cylindrical innerperipheries of the support rings are fitted to the convex-sphericalouter peripheries of the trunnions (Japanese Patent Publication No. Hei7-117108, Japanese Patent No. 2623216, etc.). The plurality of needlerollers are arranged without any retainers, or in a so-called fullcomplement state. According to this constitution, slides between thecylindrical inner peripheries of the support rings and theconvex-spherical outer peripheries of the trunnions allow the tiltingmovements of the roller assemblies including the rollers.

[0013] In constant velocity universal joints comprising rollerassemblies of this type, axial relative movements of the rollers and thesupport rings are restricted from both sides by engaging means so thatthe roller assemblies are secured in their unity as assembled articles.On the other hand, when a constant velocity universal joint of this kindtransmits rotational torque at an operating angle, tilting movements andaxial movements of the roller assemblies with respect to the trunnionsproduce slides between the inner peripheries of the support rings andthe outer peripheries of the trunnions. Then, the sliding frictionalforces therein cause axial repetitive loads (hereinafter, simplyreferred to as “axial loads”) onto the engaging means, along the axialdirections of the rollers and the support rings. Hence, the engagingmeans require such strengths as to stand the axial loads (strengthsagainst bending fatigue, cracking fatigue, and the like). Besides, theengaging means make sliding contact with the end faces of the rollersand/or the support rings and, in the cases where the rollers arerotatably supported by the support rings via needle rollers, even withthe end faces of the needle rollers. This brings about another problemof the fatigue life of those contact surfaces.

SUMMARY OF THE INVENTION

[0014] In view of the foregoing, it is an object of the presentinvention to make further reduction and stabilization of the inducedthrust and slide resistance in this kind of tripod type constantvelocity universal joint.

[0015] Another object of the present invention is to make improvementson this kind of tripod type constant velocity universal joint in therolling fatigue life of the individual component parts and theirstrengths against torsional fatigue, crack, and the like, so as toprovide a tripod type constant velocity universal joint of superiordurability and strengths while maintaining current dimensions, andprovide a tripod type constant velocity universal joint of more compactconfiguration while securing durability and strengths equivalent to orhigher than those of existing products.

[0016] Still another object of the present invention is to makeimprovements on a tripod type constant velocity universal jointcomprising roller assemblies as described above in the fatigue strengthof the engaging means, especially of the engaging rings to be attachedto the rollers/support rings, against axial loads and in the fatiguelife of their contact surfaces, so as to provide a tripod type constantvelocity universal joint of superior durability and strengths whilemaintaining its current dimensions, and provide a tripod type constantvelocity universal joint of more compact configuration while securingdurability and strengths equivalent to or higher than those of existingproducts.

[0017] To achieve the foregoing objects, the present invention providesa constant velocity universal joint comprising: an outer joint memberhaving three track grooves each having circumferentially-opposed rollerguideways; a tripod member having three radially-projecting trunnions; aroller inserted in each of the track grooves; and a support ring mountedon each of the trunnions to support the roller rotatably, the rollerbeing movable in axial directions of the outer joint member along theroller guideway, wherein the outer periphery of the roller is a partialspherical surface having the center of curvature on the trunnion axis,and the roller guideways form partial cylindrical surfaces parallel tothe axis of the outer joint member, so that the roller is capable oftilting in the track groove.

[0018] In the constitution described above, the inner periphery of thesupport ring is shaped arcuate and convex in section. The outerperiphery of each of the trunnions is shaped straight in longitudinalsection, and formed in cross section so as to make contact with theinner periphery of the support ring in a direction perpendicular to theaxis of the joint and create a clearance with the inner periphery of thesupport ring in an axial direction of the joint. The cross-sectionalconfiguration of a trunnion such as makes contact with the innerperiphery of the support ring in a direction perpendicular to the axisof the joint and creates a clearance with the inner periphery of thesupport ring in an axial direction of the joint translates into that thefaces opposed to each other in the axial direction of the tripod memberretreat toward each other, i.e., to smaller diameters than the diameterof an imaginary cylindrical surface. Concrete examples thereof includean ellipse (claims 3-5). For the sake of absorbing the tilt of thetrunnions ascribable to nutations peculiar to tripod type constantvelocity universal joints, the radius of curvature to the convex arcs ofthe support rings preferably has a value that allows the trunnions tomake a tilt of the order of 2-3°.

[0019] The trunnions may be formed to have a cross section of generallyelliptic shape with the major axis perpendicular to the axis of thejoint. The generally elliptic shape is not limited to literal ellipses,and is intended to include those generally referred to as ovals and thelike. More specifically, the configurations as set forth in claims 4-6can be adopted for the cross sections of the trunnions and the innerperipheries of the support rings so that the contact pressures againstthe support rings are relaxed and the trunnions are prevented from astrength drop. Besides, as long as the operating angle falls within apredetermined angle range, the trunnions can tilt without inclining thesupport rings. This prevents the rollers from inclination and allows therollers to roll smoothly on the roller guideways. There is provided noribs which have sometimes been arranged on the track grooves in theouter joint member with an aim to restrain the inclination of therollers. The omission of the ribs not only reduces the outer jointmember in weight and simplifies the machining thereto, but eliminatesslide resistance resulting from the slide contacts between the rollersand the ribs. This consequently achieves further reductions in slideresistance and induced thrust.

[0020] The outer periphery of each of the trunnions and the innerperiphery of the support ring may create a clearance of 0.001a orgreater in a circumferential direction of the joint, where a is thesemimajor axis of the generally elliptic cross section of the trunnion.Such clearances can well absorb the tilt of the trunnions resulting fromthe nutations of the tripod member which are peculiar to tripod typeconstant velocity universal joints. This absorption then removes thefactors responsible for the inclinations of the roller assemblies in thejoint's cross section.

[0021] The support rings may have a cylindrical inner periphery. Sincethe support rings having the cylindrical inner peripheries are mountedto the trunnions' outer peripheries having a generally elliptic crosssection, they make line contacts along the axial direction of thetrunnions with an advantageous reduction in surface pressure. In thiscase, the trunnions are limited in possible tilt angle to the supportrings. Here, the rollers are configured to be tiltable inside the trackgrooves as described above, and hence the rollers tilt at greateroperating angles while moving along the track grooves.

[0022] The trunnions may have a cylindrical outer periphery, and thegeneratrix of the inner peripheries of the support rings may comprise aconvex arc at the center. Since the spherical support rings are mountedon the trunnions' cylindrical outer peripheries, they make line contactalong the circumferential directions of the trunnions with anadvantageous reduction in surface pressure. Again, the trunnions arelimited in possible tilt angle to the support rings. The rollers areconfigured to be tiltable inside the track grooves as described above,and hence the rollers tilt at greater operating angles while movingalong the track grooves.

[0023] In the constitutions described above, a plurality of rollingelements may be interposed between the support rings and the rollers toallow relative rotations of the support rings and the rollers. Therolling elements may be needle rollers.

[0024] According to the present invention, when the joint transmitstorque with an operating angle, the tilt of the trunnions can beabsorbed into the tilt of the rollers. This contributes to a reductionin slide resistance and, finally, to a reduction in induced thrust. Theconstant velocity universal joints of the present invention areparticularly applicable to a motor vehicle's drive shaft. Thisapplication can contribute to improvements in automotive NVHperformances that depend on slide resistance and induced thrust, therebyincreasing design flexibility of portions around the car axles.

[0025] To achieve the foregoing objects, the present invention alsoprovides a constant velocity universal joint comprising: an outer jointmember having three track grooves each having circumferentially-opposedroller guideways; a tripod member having three radially-projectingtrunnions; a roller inserted in each of the track grooves; and a supportring mounted on each of the trunnions o support the roller rotatably,the roller being movable in axial directions of the outer joint memberalong the roller guideways, wherein: the support ring has a cylindricalinner periphery; and the outer periphery of each of the trunnions iscurved in longitudinal section, and formed in cross section so as tomake contact with the inner periphery of the support ring in a directionperpendicular to the axis of the joint and create a clearance with theinner periphery of the support ring in an axial direction of the joint.

[0026] The cross-sectional configuration of a trunnion such as makescontact with the inner periphery of the support ring in a directionperpendicular to the axis of the joint and creates a clearance with theinner periphery of the support ring in an axial direction of the jointtranslates into that the faces opposed to each other in the axialdirection of the tripod member retreat toward each other, i.e., tosmaller diameters than the diameter of an imaginary cylindrical surface.Concrete examples thereof include an ellipse.

[0027] Due to the changes in cross section from the conventionalcircular shape to the configuration described above, the trunnions cantilt with respect to the outer joint member without changing theorientations of the roller assemblies when the joint operates with anoperating angle. Besides, the contacting ellipses of the support ringswith the outer peripheries of the trunnions approach from oblongellipses to points in shape. This reduces the friction moments which actto tilt the roller assemblies. As a result, the roller assemblies arealways stabilized in orientation, whereby the rollers are maintainedparallel to the roller guideways for smooth rolling. This smooth rollingcontributes to a reduction in slide resistance and, finally, to areduction in induced thrust. Moreover, there is an advantage that thetrunnions improve in flexural strength due to increased section moduliat the bottom portions of the trunnions. In this connection, the innerperipheries of the support rings need not be cylindrical over the entirelengths thereof. They may be formed cylindrical only at their centersfor making contact with the trunnions, and provided with relief portionson both sides so as to avoid interference when the trunnions tilt.

[0028] The roller assemblies are interposed between the trunnions andthe outer joint member for the sake of torque transmission. In constantvelocity universal joints of this kind, the transmission direction oftorque is always perpendicular to the axis of the joint. Therefore, aslong as they contact in the transmission direction of torque, thetrunnions and the support rings can perform torque transmission withouttrouble even when they have clearances therebetween in the axialdirections of the joint.

[0029] In the above-described constitution, the trunnions may be formedto have a cross section of elliptic shape with the major axisperpendicular to the axis of the joint. The generally elliptic shapehere is not limited to literal ellipses, and is intended to includethose generally referred to as ovals and the like.

[0030] More specifically, the trunnions can adopt such cross-sectionalconfigurations as set forth in claims 14-16 so that the contactpressures against the support rings are relaxed and the trunnions areprevented from a strength drop. Besides, the trunnions can tilt withoutinclining the support rings. This prevents the rollers from inclinationand allows the rollers to roll smoothly on the roller guideways. As aresult, it becomes possible to omit ribs which are sometimes arranged onthe track grooves in the outer joint member with an aim to restrain theinclination of the rollers. The omission of the ribs not only reducesthe outer joint member in weight and simplifies the machining thereto,but eliminates slide resistance resulting from the slide contactsbetween the rollers and the ribs. As a result, further reductions inslide resistance and induced thrust are achieved.

[0031] The curve to the longitudinal sections of the trunnions may havea radius of curvature in the range of 1.1a and 8.7a. This makes itpossible to absorb the tilt of the trunnions resulting from thenutations of the tripod member which are peculiar to tripod typeconstant velocity universal joints. This absorption removes the factorsresponsible for the inclinations of the roller assemblies in the joint'scross section, and thereby contributes to improved NVH performances ofmotor vehicles.

[0032] The outer periphery of each of the trunnions may be ground onlyat a predetermined region including an area for making contact with thesupport ring. In contemplation of machining errors and the like, thepredetermined region is preferably determined to be somewhat wider thanthe contact area. The remaining portions other than the predeterminedregion may be left forge-finished without any grinding. This allows areduction in machining time and a cut in costs.

[0033] The outer periphery of the roller and the roller guideways in theouter joint member may make angular contact with each other. The angularcontact between the roller and the roller guideways makes the rollerless prone to vibrate, further stabilizing the orientation of theroller. As a result, the roller can roll on the roller guideways withsmaller resistance when moving along the axial direction of the outerjoint member. The specific constitutions to establish such angularcontact include a convex arcuate generatrix to the outer periphery ofthe roller, combined with roller guideways having a tapered or Gothicarc cross section.

[0034] In the above-described constitutions, a plurality of rollingelements can be interposed between the support rings and the rollers toallow relative rotation of the support rings and the rollers, so thatthe rollers can make smooth rotation around the trunnions for reducedslide resistance. The rolling elements may be needle rollers or balls.

[0035] According to the present invention, the trunnions can tilt withrespect to the outer joint member without changing the orientations ofthe roller assemblies when the joint operates with an operating angle.Besides, the contacting ellipses of the support rings with the outerperipheries of the trunnions approach from oblong ellipses to points inshape. This reduces the friction moments which act to tilt the rollerassemblies. As a result, the roller assemblies are always stabilized inorientation, whereby the rollers are maintained parallel to the rollerguideways for smooth rolling. This smooth rolling contributes to areduction in slide resistance and, finally, to a reduction in inducedthrust. Moreover, there is an advantage that the trunnions improve inflexural strength because of increased section moduli at the bottomportions of the trunnions.

[0036] The constant velocity universal joints of the present inventionare particularly applicable to a motor vehicle's drive shaft. Thisapplication can contribute to improvements in automotive NVHperformances that depend on slide resistance and induced thrust, therebyincreasing design flexibility of portions around the car axles.

[0037] To achieve the foregoing objects, the present invention alsoprovides a constant velocity universal joint comprising: an outer jointmember having an inner periphery provided with three axial trackgrooves, axial roller guideways being arranged on both sides of each ofthe track grooves; a tripod member having three radially-projectingtrunnions; and a roller assembly mounted on each of the trunnions of thetripod member, the roller assembly being capable of tilting movementwith respect to the trunnion and having a roller to be guided along theroller guideways in directions parallel to the axis of the outer jointmember, wherein at least one component part of the joint is limited to apredetermined range in softening resistance characteristic value (R).

[0038] The present applicant has found from a number of experiments thatthe durability of the component parts of the above-described constantvelocity universal joint, and particularly the durability of the tripodmember and the outer joint member, can be controlled accurately by usingthe softening resistance characteristic value R mentioned above.

[0039] Take the tripod member as an example. The factors affecting thedurability of the same include rolling fatigue on the outer peripheriesof the trunnions, torsional fatigue at the trunnion bottoms, andtorsional fatigue in a serration portion (or spline portion). The outerperipheries of the trunnions make rolling contact with the outerperipheries of the needle rollers, or make rolling and sliding contactwith the inner peripheries of the support rings in the rollerassemblies, and thus have the problem of rolling fatigue. The trunnionbottoms and the serration portion undergo concentrated torsionalstresses in torque transmission. This combines with the fact that theseportions are usually left unground, to give rise to the problem oftorsional stresses. Now, taking the outer joint member as an example,the factors affecting the durability thereof include rolling fatigue onthe roller guideways in the track grooves. The roller guideways makerolling and sliding contact with the outer peripheries of the rollers,and thus have the problem of rolling fatigue. Besides, the outer jointmember receives joint loads through the rollers, and hence has a problemin crack strength. Moreover, other component parts comprising the rollerassemblies also have the problem of rolling fatigue at portions to makerolling contact and/or sliding contact with their mating members.

[0040] In general, it is well known that the fatigue strengths of steelmaterial have a correlation to surface hardness. Steel materials aretherefore subjected to heat treatment and the like to form hardenedsurface layers, so that the hardened surface layers are controlled insurface hardness for required fatigue strength. The results of theexperiments made by the present applicant, however, have shown that thefatigue strengths have a closer correlation with softening resistancecharacteristics of a region ranging from the surface to a predetermineddepth (anti-softening property of the material at some highertemperatures) than with surface hardness. Then, it has been found thatthe softening resistance characteristics can be properly evaluated interms of the maximum hardness of the region within 0.5 mm in depth froma predetermined surface (softening resistance characteristic value R),and this softening resistance characteristic value R can be used as theevaluation index to the fatigue strengths. The “softening resistancecharacteristic value R” herein will be expressed as the maximum Vickershardness Hv in the region within 0.5 mm in depth from the surface, of acomponent part that is hardened and then tempered at 200° C.×2 h. Thissoftening resistance characteristic value R can be limited to apredetermined range to improve the rolling fatigue life of the componentpart and enhance its strengths against torsional fatigue and the like.

[0041] When component parts are made of steel having a carbon content of0.15-0.40% by weight and are provided with a surface layer formed bycarburizing and tempering beneath a predetermined surface, the softeningresistance characteristic value R thereof can be limited to the range of705<R≦820, and preferably 710≦R≦815, for desirable results.

[0042] When component parts are made of steel having a carbon content of0.15-0.40% by weight and are provided with a surface layer formed bycarbonitriding and tempering beneath a predetermined surface, thesoftening resistance characteristic value R thereof can also be limitedto the range of 705<R≦820, and preferably 710≦R≦815, for desirableresults.

[0043] When component parts are made of steel having a carbon content of0.45-0.60% by weight and are provided with a surface layer formed byinduction hardening and tempering beneath a predetermined surface, thesoftening resistance characteristic value R thereof can be limited tothe range of 630<R≦820, and preferably 640≦R≦810, for desirable results.

[0044] According to the present invention, the materials of thecomponent parts, notably of the tripod member and the outer jointmember, and the properties of the surfaces and subsurfaces thereof areoptimized for improvements in rolling fatigue life and in the strengthsagainst torsional fatigue and the like. This makes it possible toprovide a tripod type constant velocity universal joint of superiordurability and strengths while maintaining its current dimensions, aswell as to provide a tripod type constant velocity universal joint ofmore compact configuration while securing durability and strengthsequivalent to or higher than those of existing products.

[0045] Moreover, to achieve the foregoing objects, the present inventionprovides a constant velocity universal joint comprising: an outer jointmember having an inner periphery provided with three axial trackgrooves, axial roller guideways being arranged on both sides of each ofthe track grooves; a tripod member having three radially-projectingtrunnions; and a roller assembly mounted on each of the trunnions of thetripod member, the roller assembly being capable of tilting movementwith respect to the trunnion and having a roller to be guided along theroller guideways in directions parallel to the axis of the outer jointmember, wherein at least one component part of the joint has a surfaceportion having a residual austenite content γR (vol %) in the range of20≦γR≦40.

[0046] Generally, among typical fatigues on rolling contact surfaces isflaking (fatigue exfoliation). More specifically, it is known thatcontact surfaces subjected to repeated loads from rolling movementsgenerate cracks in their rolling portions, and these cracks develop toflaking so that the rolling fatigue life is reached. Here, a number ofexperiments and experiences have shown that the cracks that originateflaking often occur at portions somewhat inside the contact surfaces. Ithas been also found that, under such condition that metal wear chips andother foreign matters easily get into the lubricant, the contactsurfaces develop damage similar to the original flaking and reach theirrolling fatigue life because of exfoliation originating fromforeign-matter-biting indentations, peeling and smearing due toinsufficient lubricating oil films, and cracks originating therefrom(surface-origin type damage). In the latter case, the rolling fatiguelife of the contact surfaces becomes shorter than under lubricationconditions with clean lubricants.

[0047] Meanwhile, in constant velocity universal joints of this kind,the component parts have contact surfaces of greater surface roughnessas compared with those of ordinary rolling bearings. In addition, whenthe rollers make tilting movements with respect to the trunnions, aslide occurs in the contact portions between the support rings of theroller assemblies and the trunnions, or in the contact portions betweenthe inner and outer rollers of the roller assemblies. The contactportions consequently produce wear chips, which get into the lubricantand are bitten between the contact surfaces, contributing to thegeneration of indentations and the hindered formation of lubricating oilfilms with easier occurrence of the surface-origin type damage mentionedabove.

[0048] According to the present invention, at least one of the componentparts is provided with a surface layer having a residual austenitecontent γR (vol %) limited to the range of 20≦γR≦40. Therefore, thesurface layer improves in crack sensitivity so that the surface-origintype damage described above become harder to occur. Here are the reasonsfor this. That is, residual austenite is relatively low in hardness(e.g. Hv 300 or so, though depending on the carbon content of thematerial). Therefore, even if indentations are formed in the contactsurface due to the biting of foreign matters, austenite particlesdistributed in the surface portion facilitate elastic deformation aroundthe indentations, and thereby relax the stress concentration on thesurface layer and delay the propagation of crack. Besides, because ofthe deformation energy, the residual austenite undergoes a martensitetransformation for hardening. Therefore, providing the surface layerwith an appropriate amount of residual austenite can improve the surfacelayer in crack sensitivity so that the production of the surface-origintype damage described above is suppressed for enhanced rolling fatiguelife. Residual austenite contents γR of a surface layer below 20% byvolume cannot make a sufficient improvement to the crack sensitivity ofthe surface layer. On the other hand, residual austenite contents γR ofa surface layer above 40% by volume promise no further improvement tothe crack sensitivity but cause a drop in surface hardness, therebydecreasing the rolling fatigue life contrarily. Accordingly, the surfaceportion is preferably set within the range of 20≦γR≦40 in residualaustenite content γR (vol %). Incidentally, the surface layer in thepresent invention need only be formed at least beneath the contactsurface of the component part. This includes the constitution that asurface layer is formed only beneath the contact surface, theconstitution that surface layers are formed beneath the contact surfaceand surfaces adjacent thereto, and the constitution that surface layersare formed beneath the entire surfaces of the component part.

[0049] For example, at least one of the outer joint member, the tripodmember, and the component parts of the roller assemblies may be formedof steel having a carbon content of 0.15-0.40% by weight, and providedwith a carburized-and-tempered surface portion (carburized layer) or acarbonitrided-and-tempered surface portion (carbonitrided layer). Here,the residual austenite content γR (vol %) of the surface portion islimited to the range of 20≦γR≦40. According to this constitution, thesurface portion of that particular component part improves in cracksensitivity to have a structure of superior durability against rollingfatigue, while the core portion thereof forms a structure havingtoughness. As a result, the component part combines long rolling fatiguelife with crack strength and the like. This effect is particularlysignificant in the constitutions having carbonitrided-and-temperedsurface portions (carbonitrided layers). More specifically, whennitrogen is combined into a surface layer under appropriate conditions,the residual austenite and the martensite matrix become stable towardheat due to the intrusion of nitrogen. This means a structure less proneto thermal changes, with higher resistance against rolling fatigue andhigher strengths against cracks and the like. The trunnion bottoms andthe serration portion of the tripod member undergo concentratedtorsional stresses in torque transmission, and these portions areusually left unground. As a result, there occurs the problem oftorsional stresses. Nevertheless, the formation of carbonitrided layersimproves hardenability, whereby these portions are increased in surfacehardness and improved in torsional fatigue strength as well.

[0050] For example, at least one of the parts constituting the rollerassemblies may be made of steel having a carbon content of 0.95-1.10% byweight, and provided with a surface layer of nitride layer (layer havingmore solid solution of nitrogen) formed by nitriding and temperingbeneath its contact surface. Here, the residual austenite content γR(vol %) of the surface layer is limited to the range of 20≦γR≦40. As inthe constitutions described above, the surface layer of this componentpart improves in crack sensitivity to have a structure of superiorrolling fatigue strength. At the same time, the hardening uniformlyextends to the inside, advantageously decreasing deformation under highload. As a result, this component part combines longer rolling fatiguelife with higher load deformation resistance and the like.

[0051] In the constitutions described above, the softening resistancecharacteristic value R of at least either of the outer joint member andthe tripod member is desirably limited to the range of 705<R≦820, andpreferably 710≦R≦815, for the reasons stated previously.

[0052] According to the present invention, the materials of thecomponent parts and the properties of the surface layers are optimizedfor improvements of rolling fatigue life, notably of the resistanceagainst surface-origin type damage resulting from the biting of wearchips and other foreign matters. This makes it possible to provide atripod type constant velocity universal joint of superior durability andstrengths while maintaining its current dimensions, as well as toprovide a tripod type constant velocity universal joint of more compactconfiguration while securing durability and strengths equivalent to orhigher than those of existing products.

[0053] Furthermore, to achieve the foregoing objects, the presentinvention provides a constant velocity universal joint comprising: anouter joint member having an inner periphery provided with three axialtrack grooves, axial roller guideways being arranged on both sides ofeach of the track grooves; a tripod member having threeradially-projecting trunnions; and a roller assembly mounted on each ofthe trunnions of the tripod member, the roller assembly being capable oftilting movement with respect to the trunnion and having a roller to beguided along the roller guideways in directions parallel to the axis ofthe outer joint member, wherein at least one component part of thejopint has a surface portion containing a structure in which carbide isdistributed into a martensite matrix. This constitution includes thosein which only the surface layer has the above-described structure andthose in which the structure extends from the surface to the inside.

[0054] Generally, among typical fatigues on rolling contact surfaces isflaking (fatigue exfoliation). More specifically, it is known thatcontact surfaces subjected to repeated loads from rolling movementsgenerate cracks in their rolling portions, and these cracks develop toflaking so that the rolling fatigue life is reached. Here, a number ofexperiments and experiences have shown that the cracks that originateflaking often occur at portions somewhat inside the contact surfaces. Ithas been also found that, under such condition that metal wear chips andother foreign matters easily get into the lubricant, the contactsurfaces develop damage similar to the original flaking and reach theirrolling fatigue life because of exfoliation originating fromforeign-matter-biting indentations, peeling and smearing due toinsufficient lubricating oil films, and cracks originating therefrom(surface-origin type damage). In the latter case, the rolling fatiguelife of the contact surfaces becomes shorter than under lubricationconditions with clean lubricants.

[0055] Meanwhile, in constant velocity universal joints of this kind,the component parts have contact surfaces of greater surface roughnessas compared with those of ordinary rolling bearings. In addition, whenthe rollers make tilting movements with respect to the trunnions, aslide occurs in the contact portions between the support rings of theroller assemblies and the trunnions, or in the contact portions betweenthe inner and outer rollers of the roller assemblies. The contactportions consequently produce wear chips, which get into the lubricantand are bitten between the contact surfaces, contributing to thegeneration of indentations and the hindered formation of lubricating oilfilms with easier occurrence of the surface-origin type damage mentionedabove.

[0056] According to the present invention, at least the surface portionof the component part is provided with a structure in which carbide isdistributed in a martensite matrix. Therefore, the surface hardnessincreases so that contact surfaces improve in wear resistance tosuppress flaking. At the same time, foreign-matter-biting indentationsbecome hard to occur, whereby the aforementioned surface-origin typedamage are suppressed as well. This means improved rolling fatigue lifeof the contact surfaces.

[0057] The above-mentioned structure may be formed by making thecomponent part in steel material having a carbon content of 0.80% byweight or higher, e.g., in high carbon chrome steel, and subjecting thesame to dip quenching and tempering. According to this constitution, thecontact surfaces show higher resistance against flaking andsurface-origin type damage. Besides, the hardening uniformly extends tothe inside, thereby reducing deformation under high load. As a result,that particular component part combines longer rolling fatigue life withload deformation resistance and the like. The high carbon chrome steelmay use bearing steels such as SUJ1, SUJ2, SUJ3, SUJ4, and SUJ5.

[0058] The above-mentioned structure may also be formed by making thecomponent part in steel material having a carbon content of 0.15-0.40%by weight, e.g., in steel for carburization, and subjecting the same toheavy carburizing and tempering. The heavy carburizing here is a processfor increasing the content of the C solid solution in the matrix of thesurface portion to, for example, 1.5-4.0% by weight. The carbon contentof the base material, on which the core hardness depends, is preferablyset within the range of 0.15-0.40% by weight for the sake of fatiguestrengths. When the base metal has a carbon content below 0.15% byweight, the carburizing requires longer time and the core portion fallsshort of hardness as well. On the other hand, carbon contents above 0.4%by weight increase the core hardness, which significantly lowerstoughness as well as increases distortion. According to thisconstitution, the contact surfaces show higher resistance againstflaking and surface-origin type damage, while the core portion forms astructure having toughness. As a result, that particular component partcombines longer rolling fatigue life with crack strength and the like.The steel for carburization may use SCr415, SCr420, SCr430, SCr435,SCr440, SCM415, SCM420, SCM430, SCM435, SCM440, SNCM220, SNCM415,SNCM420, SNCM815, and the like. The carburizing may adopt gascarburizing or plasma carburizing. In the case of gas carburizing, thecarbon potential of the carburizing gas is increased to 1.5-4.0% byweight or higher for heavy carburizing. The plasma carburizing is aprocess in which plasma discharge of direct-current high voltage isgenerated between both electrodes in a vacuum through the medium of C inthe carburizing gas, with the furnace body as the positive electrode andthe article to be processed as the negative electrode, so that C isionized (C+) and intruded into the matrix in the surface portion of thearticle. The plasma carburizing, as a carburization undernon-equilibrium, can obtain a surface portion of higher C concentrationin a shorter time as compared with the gas carburizing. Besides, theplasma carburizing can provide a uniform concentration distribution, andthus has an advantage that an appropriate amount of carbide can beuniformly deposited in the surface portion. In this connection, when theplasma carburizing is adopted, it is preferable that the Mo and Crcontents of the steel for carburization be made higher than usual.

[0059] The deposition of carbide in the martensite matrix as describedabove allows the contact surfaces to have a surface hardness of HRC60-68, or preferably HRC 63-68. The term “HRC” herein represents C scalein Rockwell hardness. Surface hardnesses of the contact surfaces belowHRC 60 will not lead to an improvement in rolling fatigue life, whereasthose equal to or lower than HRC 68 are preferable in consideration oftoughness.

[0060] According to the present invention, the materials of thecomponent parts, or at least the structures of the surface portions areoptimized for improvements in rolling fatigue life, crack strength, thelike. This makes it possible to provide a tripod type constant velocityuniversal joint of superior durability and strengths while maintainingits current dimensions, as well as to provide a tripod type constantvelocity universal joint of more compact configuration while securingdurability and strengths equivalent to or higher than those of existingproducts.

[0061] The constant velocity universal joints according to theinventions discussed above may use roller assemblies each including theroller to be guided by the roller guideways and a support ring mountedto the outer periphery of the trunnion so as to support the rollerrotatably, wherein: the inner periphery of the support ring is shapedarcuate and convex in section; and the outer periphery of the trunnionis shaped straight in longitudinal section, and formed in cross sectionso as to make contact with the inner periphery of the support ring in adirection perpendicular to the axis of the joint and create a clearancewith the inner periphery of the support ring in an axial direction ofthe joint. In this constitution, it is the roller assemblies includingthe roller and the support ring unitarily that make tilting movementswith respect to the trunnions. Here, the tilting movements refer to thetilts the axes of the support rings and rollers make with respect to theaxes of the trunnions, within the planes containing the axes of thetrunnions. The cross-sectional configuration of a trunnion such as makescontact with the inner periphery of the support ring in a directionperpendicular to the axis of the joint and creates a clearance with theinner periphery of the support ring in an axial direction of the jointtranslates into that the faces opposed to each other in the axialdirection of the tripod member retreat toward each other, i.e., tosmaller diameters than the diameter of an imaginary cylindrical surface.Concrete examples thereof include general elliptic shapes. The “generalelliptic shapes” include those generally referred to as ovals and thelike, aside from literal ellipses.

[0062] Due to the changes in cross section from the conventionalcircular shape to the configuration described above, the trunnions cantilt with respect to the outer joint member without changing theorientations of the roller assemblies when the joint operates with anoperating angle. Besides, the contacting ellipses of the support ringswith the outer peripheries of the trunnions approach from oblongellipses to points in shape. This reduces the friction moments which actto tilt the roller assemblies. As a result, the roller assemblies arestabilized in orientation, whereby the rollers are maintained parallelto the roller guideways for smooth rolling. This smooth rollingcontributes to a reduction in slide resistance and, finally, to areduction in induced thrust.

[0063] The roller assemblies are interposed between the trunnions andthe outer joint member for the sake of torque transmission. In constantvelocity universal joints of this kind, the transmission direction oftorque is always perpendicular to the axis of the joint. Therefore, aslong as they contact in the transmission direction of torque, thetrunnions and the support rings can perform torque transmission withouttrouble even when they have clearances therebetween in the axialdirections of the joint.

[0064] In the constitutions described above, the generatrix to the innerperipheries of the support rings may comprise an arc portion at thecenter and relief portions on both sides. The arc portion preferably hassuch a radius of curvature as allows 2-3° inclination of the trunnions.Additionally, a plurality of rolling elements may be interposed betweenthe support rings and the rollers so as to allow relative rotationsbetween the support rings and the rollers. The rolling elements may beneedle rollers. Furthermore, the outer peripheries of the rollers may beformed into a spherical shape (perfect spherical surfaces or torussurfaces) so that the spherical outer peripheries of the rollers and theroller guideways of the outer joint member make angular contacts witheach other. The angular contacts between the rollers and the rollerguideways make the rollers less prone to vibrate, thereby stabilizingthe orientation of the rollers. As a result, the rollers can roll on theroller guideways with smaller resistance when moving along the axialdirection of the outer joint member. The specific configurations toestablish such angular contacts include tapered or Gothic-arc crosssections of the roller guideways.

[0065] Now, in the constant velocity universal joint of theabove-described constitution, the contact surface pressures between theouter peripheries of the trunnions and the inner peripheries of thesupport rings are higher than in the other constitutions. Therefore, theouter peripheries of the trunnions tend to have a shorter rollingfatigue life. In addition, stresses are concentrated on the bottomportions of the trunnions more easily than in the other constitutions,and hence the bottom portions tend to have lower fatigue strengths.Accordingly, it is particularly effective in the constant velocityuniversal joint of this constitution to confine the softening resistancecharacteristic values R of the outer peripheries and bottom surfaces ofthe trunnions to a predetermined range so that the outer peripheries areenhanced in rolling fatigue life and the bottom portions are enhanced intorsional fatigue strength and other strengths as described above.

[0066] Moreover, the constant velocity universal joints according to theinventions described above may use roller assemblies each including theroller to be guided by the roller guideways and a support ring fitted onthe outer periphery of the trunnion to support the roller rotatably,wherein: the trunnion has a convex-spherical outer periphery; and thesupport ring has a cylindrical or conic inner periphery. In thisconstitution, the roller assemblies including the roller and the supportring unitarily make tilting movements with respect to the trunnions.

[0067] Furthermore, the constant velocity universal joints according tothe inventions described above may use roller assemblies each includingan outer roller to be guided by the roller guideways, and an innerroller rotatably supported by the trunnion as well as fitted to theinner periphery of the outer roller, wherein: the inner roller has aconvex-spherical outer periphery; and the outer roller has an innerperiphery shaped so that a load component toward the trunnion extremityis created at a contact position with the outer periphery of the innerroller. In this constitution, the roller assemblies including the rollerand the support ring make tilting movements with respect to thetrunnions. Here, the tilting movements refer to the tilts the axes ofthe outer rollers make with respect to the axes of the trunnions, withinthe planes containing the axes of the trunnions.

[0068] To be more specific, the inner peripheries of the outer rollersmay take a variety of configurations described in Japanese PatentLaid-Open Publication No. Hei 9-14280 by the present applicant. Namely,the configurations the inner peripheries of the outer rollers may takeinclude the following: the form of a cone gradually contacting indiameter toward the trunnion extremity; a concave spherical surfacehaving a generatrix whose center falls off the center of generatrix ofthe trunnion's outer periphery toward the trunnion bottom (theconfiguration shown in FIG. 3, Japanese Patent Laid-Open Publication No.Hei 9-14280); a convex spherical surface having a generatrix whosecenter falls off the center of generatrix of the trunnion's outerperiphery toward the trunnion extremity (the configuration shown in FIG.4, Japanese Patent Laid-Open Publication No. Hei 9-14280); a compositesurface of a conical tapered surface contracting in diameter toward thetrunnion extremity and a convex spherical surface (the configurationshown in FIG. 5, Japanese Patent Laid-Open Publication No. Hei 9-14280);and a composite surface of a cylindrical surface and a convex sphericalsurface (the configuration shown in FIG. 9, Japanese Patent Laid-OpenPublication No. Hei 9-14280). Nevertheless, in favor of simplifiedfabrication processes, the inner peripheries of the outer rollerspreferably have the form of a cone gradually contracting in diametertoward the trunnion extremity. In that case, the inner peripheries ofthe outer rollers desirably have a tilt angle of 0.1-3°, and preferably0.1-1°, for the sake of effective reduction and stabilization of theinduced thrusts.

[0069] In the constitutions described above, many minute dimples may beformed randomly at least in the contact surfaces including the outerperipheries of the trunnions and the roller guideways. The minutedimples formed in the contact surfaces function as oil sumps to promotethe formation of oil films on the contact surfaces, thereby improvingthe lubricity and enhancing the rolling fatigue life of the contactsurfaces. For example, the minute dimples have a size of several tens ofμm or so, and a depth of 1 μm or so. Grinding conditions to the contactsurfaces can be changed to form minute dimples of arbitrary size, depth,and number. Incidentally, when it is difficult to form minute dimplesselectively in the contact surfaces alone, minute dimples may also beformed in the vicinities of the contact surfaces of that component part,or over the entire surfaces.

[0070] A solid lubrication coating may be formed on the contact surfacesincluding the outer peripheries of the trunnions and the rollerguideways, with a chemical conversion coating as an undercoating. Sincethe solid lubrication coating reduces the frictional resistance on thecontact surfaces and improves the lubricity, the contact surfacesimprove in rolling fatigue life. The chemical conversion coating to bethe undercoating is formed with the objective of enhancing the solidlubrication coating in adhesion to the contact surfaces. Examples of thechemical conversion coating include a manganous phosphate coating, aniron phosphate coating, and a zinc phosphate coating. Examples of thesolid lubrication film include a molybdenum disulfide coating and a PTFEcoating. In this connection, the effect after the treatment depends onthe pre-treatment surface roughness of the contact surfaces(base-material surfaces). It is therefore desirable that the contactsurfaces be previously finished with a surface roughness of Ra 0.2-0.8,for the sake of appropriate oil-sump functions. In the cases where theselective application of coating to the contact surfaces alone isdifficult, the coating may also be applied to the vicinities of thecontact surfaces of those component parts, or over the entire surfaces.

[0071] Cold sulfurizing may be applied to the contact surfaces includingthe outer peripheries of the trunnions and the roller guideways.Sulfurizing is a surface treating method for infiltrating sulfur to thesurface of steel to generate iron sulfide. The application ofsulfurizing reduces the frictional resistance on the surface; therefore,the surface improves in initial conformability for enhanced rollingfatigue life and stabilized NVH performances as well. Since the coldsulfurizing is performed under such a condition as 30-40° C.×10-30 min.,no hardness drop occurs in the surface hardened layers. The effect afterthe treatment depends on the pre-treatment surface roughness of thecontact surfaces (base-material surfaces). Thus, it is desirable thatthe contact surfaces be previously finished with a surface roughness ofRa 0.2-0.8 for the sake of appropriate oil-sump functions.

[0072] Moreover, to achieve the foregoing objects, the present inventionprovides a constant velocity universal joint comprising: an outer jointmember having an inner periphery provided with three axial trackgrooves, axial roller guideways being arranged on both sides of each ofthe track grooves; a tripod member having three radially-projectingtrunnions; and a roller assembly mounted on each of the trunnions of thetripod member, the roller assembly including a roller to be guided alongthe roller guideways in directions parallel to the axis of the outerjoint member, a support ring for supporting the roller rotatably, andengaging means for retaining the roller and the support ring from bothsides so as to prevent axial relative movement of the roller and thesupport member, the roller assembly being capable of tilting movementsand axial displacements with respect to the trunnion, wherein at leasteither one of the engaging means has a engaging ring attached to theroller or the support ring, the engaging ring having a width W in therange of 0.5 mm≦W≦1.2 mm and a surface hardness in the range of HRC 43to HRC 52.

[0073] Here, the constitution that “at least either one of the engagingmeans has a engaging ring attached to the roller or the support ring”includes the constitutions in which one of the engaging means is aengaging ring and the other engaging means consists of a engaging collarintegrally arranged on the roller or the support ring, and theconstitutions in which both of the engaging means are engaging rings. Italso covers such a constitution that at least one of the engaging meansconsists of the engaging ring and another engaging element, e.g., of theengaging ring and a engaging collar. Furthermore, the term “engagingring” includes not only solid support rings having perfect support ringshapes but also split rings partially split by a slit.

[0074] The engaging rings are set within the range of 0.5 mm≦W≦1.2 mm inwidth W for the following reason. The engaging rings, as describedpreviously, undergo repeated axial loads via the rollers (or the supportrings) and the needle rollers. It is thus essential to provide theengaging rings with appropriate toughness for the sake of highercapacity for the axial loads and higher fatigue strengths. That is, theprovision of appropriate toughness for the engaging rings disperses theaxial loads imposed on the engaging rings, resulting in an improvementin the fatigue strengths of the engaging rings. Besides, in constantvelocity universal joints of this kind, engaging rings are oftenattached to the rollers or the support rings as contracted/expanded indiameter. Therefore, the provision of appropriate toughness to theengaging rings is also desirable in terms of mountability. Furthermore,in favor of simplified fabrication processes, consideration is desirablygiven to the workability of the engaging rings. The setting of theengaging rings within the range of 0.5 mm≦W≦1.2 mm in width W canprovide appropriate toughness for the engaging rings, so that they areimproved in the fatigue strength against axial loads and enhanced in themountability to the rollers or support rings at the same time. Here, theengaging rings also improve in workability.

[0075] Meanwhile, in favor of higher fatigue strength against the axialloads and enhanced fatigue life of the contact surfaces, it is desirablethat the surfaces of the engaging rings be provided with appropriatehardness for excellent wear resistance. It is for this reason that theengaging rings are set within the range of HRC 43 to HRC 53 in surfacehardness. The term “HRC” here represents C scale in Rockwell hardness.Surface hardnesses below HRC 43 cannot provide the contact surfaces withsufficient fatigue life. Surface hardnesses above HRC 53 cause a drop intoughness, which is unfavorable in views of fatigue strength againstaxial loads and in terms of mountability.

[0076] In the constitution described above, at least surface layers ofthe engaging rings may contain a structure in which spheroidized carbideis distributed into a martensite matrix. Here, the phrase “at leastsurface layers of the engaging rings contain a structure in whichspheroidized carbide is distributed into a martensite matrix” coverssuch constitutions that only the surface layers contain theabove-mentioned structure, and that the above-mentioned structureextends from the surfaces to the insides.

[0077] According to this constitution, at least the surface layers ofthe engaging rings are provided with the structure with martensitematrix containing spheroidized carbide. This yields a wear resistancehigher than those of steels for general structure, thereby improving thecontact surfaces in fatigue life.

[0078] The above-mentioned carbide consists mainly of Fe₃C. Thestructure having such carbide distributed into its martensite matrix canbe formed by providing at least the surface layers with carbon C as muchas or more than its eutectic point (0.8% by weight or higher), andsubjecting the same to hardening and tempering.

[0079] To be more specific, the engaging rings may be made of carbontool steel, and the martensite matrix be provided with a spheroidizedcarbide content of 0.3-0.6% by weight. According to this constitution,the martensite matrix contains an appropriate amount offine-spheroidized carbide, and therefore achieves higher wearresistance. In the meantime, the core portion is prevented from anexcessive increase in hardness, and thus forms a structure ofappropriate toughness. As a result, the contact surfaces of the engagingrings improve in fatigue life, as well as in the fatigue strengthagainst axial loads. Moreover, since the engaging rings secure anappropriate toughness, they are also enhanced in the mountability to therollers or support rings. Here, the martensite matrix is preferablylimited to the range of 0.3-0.6% by weight in spheroidized carbidecontent. Spheroidized carbide contents below 0.3% by weight cannotproduce the effect of improving the wear resistance sufficiently. Incontrast, spheroidized carbide contents above 0.6% by weight can makethe matrix so low in toughness as to fall short of the fatigue strengthagainst axial loads and the mountability. The carbon tool steel may useSK3, SK4, SK5, SK6, and the like.

[0080] Otherwise, the engaging rings may be made of spring steel.According to this constitution, higher elastic limits can be achievedwhile maintaining high surface hardness. Therefore, the contact surfacesof the engaging rings improve in fatigue life as well as in the fatiguestrength against axial loads. Moreover, the achievement of higherelastic limits further improves the engaging rings in mountability,which is also effective in automating the mounting process and therebyreducing the fabrication costs. The spring steel can be selected andused irrespective of type; an optimum one may be selected from amonghot-forming spring steels and cold-forming spring steels in accordancewith use conditions, joint size, and the like. For example, hot-formingspring steel SUP4 and the like may be used.

[0081] The engaging rings may also be made from a hard steel wire rod.Although slightly inferior in wear resistance as compared with theconstitutions described above, this constitution provides higher elasticlimits, and thereby disperses the axial loads imposed on the engagingring. As a result, higher fatigue strengths against axial loads areobtained. Besides, hard steel wire rods are relatively inexpensive aswell as effective at improving mountability. For example, the hard steelwire rod may use SWRH or the like.

[0082] In the above-described constitutions, the engaging rings arepreferably attached to the rollers or the support rings with no play.The phrase “with no play” here refers to a state in which the engagingrings are mounted to the rollers or the support rings at least with noradial play. Elimination of axial play as well as the radial play ispreferable. According to this constitution, the no-play attachment ofthe engaging rings to the rollers or the support rings stabilizes theareas of action (the load points) of the axial loads the engaging ringsreceive from the rollers or the support rings. This results in enhancedfatigue strength against axial load. Besides, the suppression of theload-point fluctuations also improves the fatigue life of the contactsurfaces between the engaging rings and the rollers or the supportrings.

[0083] In addition, the other of the engaging means may be composed of aengaging collar formed integrally on a roller or a support rings so thatassembling tolerance due to the attachment of a engaging ring to thisportion is eliminated. As a result, the axial clearances from theengaging means on both sides to the roller or the support ring can bereduced by half. This can make the above-described effects moresignificant.

[0084] In the constitutions described above, many minute dimples may beformed randomly at least in the contact surfaces of the engaging means(engaging rings and/or engaging collars). The minute dimples formed inthe contact surfaces function as oil sumps to promote the formation ofoil films on the contact surfaces, improving the lubricity and enhancingthe rolling fatigue life of the contact surfaces. For example, theminute dimples have a size of several tens of μm or so, and a depth of 1μm or so. Grinding conditions to the contact surfaces can be changed toform minute dimples of arbitrary size, depth, and number. When it isdifficult to form minute dimples selectively in the contact surfacesalone, minute dimples may also be formed in the vicinities of thecontact surfaces or over the entire surfaces of the engaging rings andthe rollers/support rings.

[0085] A solid lubrication coating may be formed at least on the contactsurfaces of the engaging means (the engaging rings and/or the engagingcollars), with a chemical conversion coating as an undercoating. Sincethe solid lubrication coating reduces the frictional resistance on thecontact surfaces and improves the lubricity, the contact surfacesimprove in fatigue life. The chemical conversion coating to be theundercoating is formed with the objective of increasing the solidlubrication coating in adhesion to the contact surfaces. Examples of thechemical conversion coating include a manganous phosphate coating, aniron phosphate coating, and a zinc phosphate coating. Examples of thesolid lubrication coating include a molybdenum disulfide coating and aPTFE coating. In this connection, the effect after the treatment dependson the pre-treatment surface roughness of the contact surfaces(base-material surfaces). It is therefore desirable that the contactsurfaces be previously finished with a surface roughness of Ra 0.2-0.8for the sake of appropriate oil-sump functions. In the cases where theselective application of coating to the contact surfaces alone isdifficult, the coating may also be applied to the vicinities of thecontact surfaces, or over the entire surfaces of the engaging rings andthe rollers/support rings.

[0086] Cold sulfurizing may be applied at least to the contact surfacesof the engaging means (the engaging rings and/or the engaging collars).Sulfurizing is a surface treating method for infiltrating sulfur to thesurface of steel to generate iron sulfide. The application ofsulfurizing reduces the frictional resistance on the surface; therefore,the surface improves in initial conformability for enhanced rollingfatigue life and stabilized NVH performances as well. Since the coldsulfurizing is performed under such a condition as 30-40° C.×10-30 min.,no hardness drop occurs in the surface hardened layers. The effect afterthe treatment depends on the pre-treatment surface roughness of thecontact surfaces (base-material surfaces). Thus, it is desirable thatthe contact surfaces be previously finished with a surface roughness ofRa 0.2-0.8 for the sake of appropriate oil-sump functions. In the caseswhere the selective application of sulfurizing to the contact surfacesalone is difficult, the sulfurizing may also be applied to thevicinities of the contact surfaces, or over the entire surfaces of theengaging rings and the rollers/support rings.

[0087] Shot peening may be applied at least to the contact surfaces ofthe engaging means (the engaging rings and/or the engaging collars).With the conditions including the size of the shot particles, the speedof shot, and the amount of shot adjusted appropriately, minute dimplescan be formed in the contact surfaces so that the minute dimples havethe oil sump functions for improved lubricity. The application of theshot peening produces finer surface structures as well as causesresidual compressive stress on the surfaces. Therefore, the shot peeningis effective at improving the fatigue strength against axial loads andthe fatigue life of the contact surfaces. In the cases where theselective application of shot peening to the contact surfaces alone isdifficult, the shot peening may also be applied to the vicinities of thecontact surfaces, or over the entire surfaces of the engaging rings andthe rollers/support rings.

[0088] The constant velocity universal joints according to the presentinvention may use roller assemblies each including a roller to be guidedby the roller guideways and the support ring mounted on the outerperiphery of the trunnion to support the roller rotatably, wherein: theinner periphery of the support ring is shaped arcuate and convex insection; and the outer periphery of the trunnion is shaped straight inlongitudinal section, and formed in cross section so as to make contactwith the inner periphery of the support ring in a directionperpendicular to the axis of the joint and create a clearance with theinner periphery of the support ring in an axial direction of the joint.Otherwise, the constant velocity universal joints according to thepresent invention may use roller assemblies each including a roller tobe guided by the roller guideways and a support ring fitted on the outerperiphery of the trunnion to support the roller rotatably, wherein: thetrunnion has a convex-spherical outer periphery; and the support ringhas a cylindrical or conical inner periphery. Since the details of theseconstitutions are identical to those described previously, descriptionthereof will be omitted.

[0089] According to the present invention, the engaging means,especially the engaging rings to be attached to the rollers/supportrings, improve in the fatigue strength against axial loads and in thefatigue life of their contact surfaces. This makes it possible toprovide a tripod type constant velocity universal joint of superiordurability and strengths while maintaining its current dimensions, aswell as to provide a tripod type constant velocity universal joint ofmore compact configuration while securing durability and strengthsequivalent to or higher than those of existing products.

[0090] The nature, principle, and utility of the invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by like reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] In the accompanying drawings:

[0092] FIGS. 1(A)-1(C) show a tripod type constant velocity universaljoint according to a first embodiment of the present invention, FIG.1(A) being a partially-sectioned end view of the same, FIG. 1(B) asectional view perpendicular to a trunnion, and FIG. 1(C) a sectionalview of a support ring;

[0093]FIG. 2(A) is a longitudinal sectional view showing the constantvelocity universal joint of FIGS. 1(A)-1(C) with an operating angle, andFIG. 2(B) is a schematic side view of the tripod member in FIG. 2(A);

[0094]FIG. 3 is an enlarged sectional view of a support ring;

[0095]FIG. 4 (A) is a longitudinal sectional view of the constantvelocity universal joint, showing the relationship between a trunnionand a roller assembly, and FIG. 4(B) is a plan view of the trunnion andthe roller assembly;

[0096]FIG. 5 is a cross-sectional view of a trunnion;

[0097]FIG. 6(A) is a sectional view taken along the axial direction of atrunnion, showing the trunnion and a roller assembly, and FIG. 6(B) is asectional view perpendicular to the trunnion, showing the trunnion and asupport ring;

[0098]FIG. 7(A) is a sectional view taken along the axial direction of atrunnion, showing the trunnion and a roller assembly, and FIG. 7(B) is asectional view perpendicular to the trunnion, showing the trunnion and asupport ring;

[0099] FIGS. 8(A) and 8(B) show a tripod type constant velocityuniversal joint according to a second embodiment of the presentinvention, FIG. 8(A) being a partially-sectioned end view of the sameand FIG. 8(B) a sectional view perpendicular to a trunnion;

[0100]FIG. 9(A) is a longitudinal sectional view showing the constantvelocity universal joint of FIGS. 8(A) and 8(B) with an operating angle,and FIG. 9(B) is a schematic side view of the tripod member in FIG.9(A);

[0101]FIG. 10 is an enlarged sectional view of a support ring;

[0102]FIG. 11(A) is a sectional view of a tripod member and a rollerassembly, and FIG. 11(B) is a plan view of the same;

[0103] FIGS. 12(A) and 12(B) show a tripod type constant velocityuniversal joint according to a third embodiment of the presentinvention, FIG. 12(A) being a partially-sectioned end view of the same,and FIG. 12(B) a sectional view perpendicular to a trunnion;

[0104]FIG. 13 is a longitudinal sectional view showing the constantvelocity universal joint with an operating angle;

[0105]FIG. 14 is an enlarged sectional view of a support ring;

[0106]FIG. 15 is a cross-sectional view of a trunnion;

[0107]FIG. 16 is a cross-sectional view of a trunnion;

[0108]FIG. 17 is a cross-sectional view of a trunnion;

[0109] FIGS. 18(A)-18(C) show a tripod type constant velocity universaljoint according to a fourth embodiment of the present invention, FIG.18(A) being a partially-sectioned end view of the same, FIG. 18(B) asectional view perpendicular to a trunnion in FIG. 18(A), and FIG. 18(C)a sectional view of a support ring for explaining a contacting ellipse;

[0110]FIG. 19(A) is a longitudinal sectional view showing the constantvelocity universal joint of FIGS. 18(A)-18(C) with an operating angle,and FIG. 19(B) is a schematic side view of the tripod member in FIG.19(A);

[0111] FIGS. 20(A)-20(C) show a tripod type constant velocity universaljoint according to a fifth embodiment of the present invention, FIG.20(A) being a partially-sectioned end view of the same, FIG. 20(B) asectional view perpendicular to a trunnion in FIG. 20(A), and FIG. 20(C)a longitudinal sectional view showing the joint with an operating angle;

[0112]FIG. 21 is an enlarged sectional view of a support ring in FIGS.20(A)-20(C);

[0113] FIGS. 22(A) and 22(B) show a tripod type constant velocityuniversal joint according to a sixth embodiment of the presentinvention, FIG. 22(A) being a partially-sectioned end view of the same,and FIG. 22(B) an enlarged cross-sectional view of the essential partsin FIG. 22(A);

[0114]FIG. 23 is a diagram explaining a load component F occurring atthe contact position between the support ring and the trunnion in FIGS.22(A) and 22(B);

[0115] FIGS. 24(A)-24(C) are a tripod type constant velocity universaljoint according to a seventh embodiment of the present invention, FIG.24(A) being a cross-sectional view of the same,

[0116]FIG. 24(B) an enlarged cross-sectional view of the essential partsin FIG. 24(A), and FIG. 24(C) a diagram explaining a load component Foccurring at the contact position between an outer roller and an innerroller;

[0117]FIG. 25 is a chart showing conditions for plasma carburizing;

[0118]FIG. 26 is an enlarged partial sectional view of a rollerassembly;

[0119]FIG. 27 is an enlarged partial sectional view of a roller assemblyaccording to a modified example;

[0120]FIG. 28(A) is an enlarged partial sectional view of a rollerassembly according to another modified example, and FIG. 28(B) is anenlarged view of X region in FIG. 28(A);

[0121]FIG. 29(A) is an enlarged partial sectional view of a rollerassembly according to another modified example, and FIG. 29(B) is anenlarged view of Y region in FIG. 29(A);

[0122]FIG. 30 is a partial sectional view showing a engaging ring;

[0123]FIG. 31 is an enlarged partial sectional view of a roller assemblyaccording to another modified example;

[0124]FIG. 32 is an enlarged partial sectional view of a roller assemblyaccording to another modified example;

[0125]FIG. 33 is an enlarged partial sectional view of a roller assemblyaccording to another modified example; and

[0126] FIGS. 34(A)-34(D) are partial sectional views showing end facesof needle rollers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0127] Hereinafter, description will be given of the embodiments of thepresent invention.

[0128] FIGS. 1(A) through 2(B) shows a tripod type constant velocityuniversal joint according to a first embodiment of the presentinvention. FIG. 1(A) shows an end face of the joint (partiallysectioned), and FIG. 2(A) shows a longitudinal section of the joint atan operating angle of θ. The constant velocity universal joint ischiefly composed of an outer joint member 10 and a tripod member 20. Theouter joint member 10 is connected to one of two shafts to be coupled,and the tripod member 20 is to the other.

[0129] The outer joint member 10 has three track grooves 12 axiallyextending in its inner periphery. Each of the track grooves 12 hasroller guideways 14 formed on its circumferentially-opposed side walls.The tripod member 20 has three trunnions 22 projecting radially. Each ofthe trunnions 22 carries a roller 34, and this roller 34 is accommodatedin one of the track grooves 12 in the outer joint member 10. In eachtrack groove 12, the roller guideways 14 opposed to each other in thecircumferential direction of the joint form part of a cylindricalsurface parallel to the axis of the outer joint member 10. The outerperiphery of each roller 34 is a partial spherical surface with thecenter of curvature on the axis of the trunnion 22. Accordingly, therollers 34 are tiltable in the track grooves 12. An annular support ring32 is fitted onto the outer periphery of each trunnion 22. This supportring 32 and the roller 34 are unitized via a plurality of needle rollers36 to constitute a roller assembly capable of relative rotationstherebetween. More specifically, the needle rollers 36 are rotatablyinterposed between inner and outer raceway surfaces, with thecylindrical outer periphery of the support ring 32 and the cylindricalinner periphery of the roller 34 as the inner and outer racewaysurfaces, respectively. As shown in FIG. 1(B), the needle rollers 36 arearranged in a so-called full complement state, where the rollers areloaded in as many as possible without any retainer. The referencenumerals 33 and 35 designate a pair of washers which are fitted toannular grooves formed in the inner periphery of each roller 34, with anaim to stop the needle rollers 36 from coming off. Each of the washers33, 35 has a cut across its circumferential direction (see FIG. 4(B)) soas to be fitted to the annular groove in the inner periphery of theroller 34 as elastically contracted in diameter.

[0130] In longitudinal section (FIGS. 1(A) and 2(A)), each trunnion 22has an outer periphery of straight shape, parallel to the axis of thetrunnion 22. In cross section (FIG. 1(B)), the trunnion 22 has agenerally ellipse shape with the major axis orthogonal to the axis ofthe joint. The cross section of the trunnion 22 is generally elliptic inshape, with a shrinkage in the non-load direction, or in thickness asseen in the axial direction of the tripod member 20, when compared withthe load direction. In other words, each trunnion 22 has such across-sectional configuration that the faces opposed to each other inthe axial direction of the tripod member 20 retreat toward each other,i.e., to smaller diameters than the diameter of the imaginarycylindrical surface.

[0131] The inner periphery of each support ring 32 is arcuate and convexin section. That is, the generatrix to the inner periphery is a convexarc having a radius of r (FIG. 1(C)). This combines with theabove-described general elliptic cross sections of the trunnions 22 andthe provision of predetermined clearances between the trunnions 22 andthe support rings 32, to allow the support rings 32 to move along theaxial directions of the trunnions 22 as well as make tilting movementswith respect to the trunnions 22. In addition, the support rings 32 andthe rollers 34 are unitized via the needle rollers 36 so as to becapable of relative rotations as described above. Therefore, the supportrings 32 and rollers 34 are capable of unitary tilting movements withrespect to the trunnions 22. Here, the term “tilting movements” refersto the tilts the axes of the support rings 32 and rollers 34 make withrespect to the axes of the trunnions 22, within the planes containingthe axes of the trunnions 22 (see FIG. 2(A)).

[0132] In conventional joints, trunnions make contact with the innerperipheries of support rings at the full lengths of their outerperipheries. This produces circumferentially extended contactingellipses. Therefore, when the trunnions tilt with respect to the outerjoint member, there arise friction moments which function to tilt thesupport rings, and finally the rollers, with the movement of thetrunnions. Meanwhile, in the embodiment shown in FIGS. 1(A)-1(C), thegenerally elliptic cross sections of the trunnions 22 and the innerperipheries of the support rings 32 whose generatrixs are convex arcswith radius r make the contacting ellipses closer to points as shown bythe broken line in FIG. 1(C), with a reduction in area at the same time.As a result, the forces to tilt the roller assemblies (32, 34) decreasegreatly as compared to the conventional ones, with a further improvementin the orientation stability of the rollers (34).

[0133] As shown in FIG. 3, each support ring 32 may comprise acombination of an arc portion at the center and relief portions 32 b onboth sides. The role of the relief portions 32 b is to avoid theinterference with the trunnion 22 at an operating angle of θ as shown inFIG. 2(A). Each relief portion 32 b is formed by a straight or curvedline that gradually spreads out in diameter from an edge of the arcportion 32 a to an end of the support ring 32. The relief portions 32 billustrated here are formed as part of a conical surface having a vertexangle α=50°. The arc portion 32 a has a large radius of curvature, forexample, of the order of 30 mm so that the trunnion 22 can make a tiltof 2-3° or so with respect to the support ring 32.

[0134] In tripod type constant velocity universal joints, one rotationof the outer joint member 10 constitutionally produces three nutationsof the tripod member 20 with respect to the center of the outer jointmember 10. Here, the amount of eccentricity represented by the referencesymbol e (in FIG. 2(A)) increases in proportion to the operating angleθ. While the three trunnions 22 are spaced by 120° from one another, theoperating angle θ causes the trunnions 22 to tilt as shown in FIG. 2(B).More specifically, with reference to the vertical trunnion 22 shown tothe upper in the diagram, the remaining two trunnions 22 are declinedslightly from their zero-operation-angle axes shown by the dot-dashlines. For example, an operating angle θ of approximately 23° causes adecline of the order of 2-3°. This decline can be readily allowed by thecurvature of the arc portions 32 a on the inner peripheries of thesupport rings 32. Therefore, the surface pressures at the contactportions between the trunnions 22 and the support rings 32 can beprevented from increasing excessively. FIG. 2(B) is a schematicrepresentation of the three trunnions 22 of the tripod member 20 as seenfrom the left side of FIG. 2(A), the full lines showing the individualtrunnions. The major axis 2 a of each trunnion 22 and the inner diameterof the corresponding support ring 32 create a clearance for absorbingthe tilt of the trunnion 22 resulting from the nutation of the tripodmember 20. Examples of specific figures to this clearance are listed inTable 1. TABLE 1 MINIMUM r: OPTIMUM VALUE CLEARANCE FOR OPERATING(MINIMUM SURFACE NUTATION ANGLE θ (°) PRESSURE) ABSORPTION 15  2.898a1.131 × 10⁻³a 10  4.731a 0.330 × 10⁻³a  5 10.392a 0.041 × 10⁻³a

[0135] In the constant velocity universal joint of the presentembodiment, relaxation of surface pressures is required due to the factthat the trunnions 22 having the generally elliptic cross sections andthe support rings 32 having the circular cross sections make contactwith each other for torque transmission. Hereinafter, this point will beexplained with reference to FIGS. 4(A) and 4(B). Incidentally, thevertical direction in FIG. 4(B) represents the load direction, and thehorizontal the non-load direction.

[0136] When the joint transmits torque at an operating angle of θ, eachtrunnion 22 makes reciprocating movements with respect to its supportring 32 within the bounds of the operating angle θ, as shown by thebroken lines in FIG. 4(A). Here, in the non-load direction, the trunnion22 and the support ring 32 have a relatively large clearance, whichallows the trunnion 22 to swing without interfering with the supportring 32. In the load direction, however, the trunnion 22 increases inapparent curvature as shown by the broken line in FIG. 4(B) as theoperating angle θ widens to increase the tilt of the trunnion 22. Whenthe apparent curvature exceeds the curvature of the inner diameter ofthe support ring 32, the trunnion 22 comes into two-point contact withthe support ring 32. Then, the trunnion 22 cannot tilt freely by itself,and starts to involve the support ring 32, and finally the rollerassembly (32, 34), in its inclination. On this account, thecross-sectional constitution of the trunnions 22, especially thedimensions in the load direction, are determined so that the trunnions22 can tilt within a predetermined angle range by themselves withoutinterfering with the support rings 32.

[0137] Specifically, assuming that the maximum operating angle θ max is25°, the setting that allows the joint to take the maximum operatingangle without tilting the support rings 32 and minimize the surfacepressures between the trunnions 22 and the support rings 32 is asfollows:

[0138] r=1.369a

[0139] b/a=0.759,

[0140] where a and b are the semimajor and semiminor axes of thegenerally elliptic cross section of a trunnion 22, respectively (seeFIG. 5), and r is the radius of curvature of the inner periphery of asupport ring (see FIGS. 1(C) and 3). Given that the radius of curvaturer of the support ring inner periphery has a recommendable range of 0.5rand 1.5r, i.e., 0.684a and 2.053a, the ellipticity b/a falls within therange of 0.836 and 0.647.

[0141] Although feasible in terms of configuration, the above-describedsetting may cause too high a surface pressure between the trunnions 22and the support rings 32 to make practical use of the joint for motorvehicles. Therefore, in the cases where lower vibrations are desiredunder the range of normal operating angles in automotive applications,the non-tilting angles of the roller assemblies (32, 34) can be loweredto reduce the surface pressures and allow the practical use of thejoint. For example, Table 2 lists optimum values and recommendableranges for the radius of curvature r of the support ring's innerperiphery and the ellipticity b/a, assuming that the normal operatingangle 0 is greater than 5° and smaller than 15°. TABLE 2 OPTIMUM VALUE(MINIMUM RECOMMENDABLE NO-ROLLER- SURFACE RANGE TILT ANGLE PRESSURE) 0.5r 1.5 r r 15  2.898a 1.449a  4.347a 10  4.731a 2.365a  7.096a 5 10.392a5.196a 15.588a b/a 15  0.859 0.914  0.801 10  0.909 0.948  0.869 5 0.956 0.976  0.935

[0142] The above-described embodiment is built on the combination of thetrunnions 22 having general arc cross sections and the support rings 32having the convex arc inner peripheries. However, other combinations maybe adopted instead. For example, as shown in FIGS. 6(A)-7(B), trunnions22 and support rings 32 may be put into line contact with each other forlower surface pressures. In the embodiment of FIGS. 6(A) and 6(B), asupport ring 32 having a cylindrical inner periphery is fitted onto atrunnion 22 having an elliptic cross section. Here, the two members arein line contact with each other along the axial direction. In theembodiment of FIGS. 7(A) and 7(B), a support ring 32 having a convex arcinner periphery is fitted onto a trunnion 22 having a cylindrical outerperiphery. Here, the two members are in line contact with each otheralong the circumferential direction. Both of these constitutions aremade feasible by the capability of the rollers 34 to tilt inside thetrack grooves 12. More specifically, the tilt angle the trunnions 22 cantake with respect to the support rings 32 is limited, and hence theroller assemblies (32, 34) come to tilt when the joint transmits torqueat an operating angle. Then, this tilt is allowed by the rollers 34tilting inside the track grooves 12.

[0143] FIGS. 8(A) through 9(B) show a tripod type constant velocityuniversal joint according to a second embodiment of the presentinvention. Here, FIG. 8(A) is an end view of the joint (partiallysectioned). FIG. 8(B) shows a section perpendicular to a trunnion. FIG.9(A) shows a longitudinal section of the joint at an operating angle ofθ. The constant velocity universal joint is chiefly composed of an outerjoint member 10 and a tripod member 20. The outer joint member 10 isconnected to one of two shafts to be coupled, and the tripod member 20is connected to the other.

[0144] As shown in FIGS. 8(A) and 8(B), the outer joint member 10 hasthree track grooves 12 axially extending in its inner periphery. Each ofthe track grooves 12 has roller guideways 14 formed on itscircumferentially-opposed side walls. The tripod member 20 has threetrunnions 22 projecting radially. A roller 34 is attached to each of thetrunnions 22, and this roller 34 is accommodated in one of the trackgrooves 12 in the outer joint member 10. The roller 34 has an outerperiphery having a sectional configuration conforming to the rollerguideways 14.

[0145] The outer periphery of each roller 34 forms a convex surfacewhose generatrix is an arc having the center of curvature radially offthe axis of the trunnion 22. The roller guideways 14 have a section ofGothic arc shape. Thus, the roller 34 and the roller guideways 14 makeangular contacts with each other. In FIG. 8(A), dot-dash lines show thetwo contact positions. Although omitted from the drawings, the sphericalouter periphery of the roller may be combined with tapered crosssections of the roller guideways 14 to achieve angular contactstherebetween. The adoption of such constitutions as provide angularcontacts between the rollers 34 and the roller guideways 14 makes therollers less prone to vibrate, thereby stabilizing the orientations ofthe rollers. Incidentally, when angular contacts are not adopted, theroller guideways 14 may comprise, for example, by part of a cylindricalsurface whose axis is parallel to that of the outer joint member 10. Inthis case, the cross-sectional configuration of the guideways 14 may bean arc corresponding to the generatrix to the outer peripheries of therollers 34.

[0146] A support ring 32 is fitted onto the outer periphery of eachtrunnion 22. This support ring 32 and the roller 34 are unitized via aplurality of needle rollers 36 to constitute a roller assembly capableof relative rotations therebetween. More specifically, the needlerollers 36 are rotatably interposed between inner and outer racewaysurfaces, with the cylindrical outer periphery of the support ring 32and the cylindrical inner periphery of the roller 34 as the inner andouter raceway surfaces, respectively. As shown in FIG. 8(B), the needlerollers 36 are arranged in a so-called full complement state, where therollers are loaded in as many as possible without any retainer. Thereference numerals 33 and 35 represent a pair of washers which arefitted to annular grooves formed in the inner periphery of each roller34, with an aim to stop the needle rollers 36 from coming off. Thesewashers 33, 35 have a cut across their circumferential directions (seeFIG. 11(B)), so that they are inserted into the rollers 34 aselastically contracted in diameter and then fitted to the annulargrooves with the aid of their elastic expanding forces.

[0147] In longitudinal section (FIG. 8(A) or 9(A)), each trunnion 22 hasan outer periphery comprising a convex curve that bulges outward at thecenter, or a convex arc having a radius of curvature of R for example.In cross section (FIG. 8(B)), the trunnion 22 has the form of an ellipsewhose major axis is orthogonal to the axis of the joint. In other words,each trunnion has such a cross-sectional configuration that the facesopposed to each other in the axial direction of the tripod member 20retreat toward each other, i.e., to smaller diameters than the diameterof the imaginary cylindrical surface.

[0148] As shown in FIG. 10, each support ring 32 has a cylindrical innerperiphery. This combines with the above-described, convex-curvedlongitudinal sections of the trunnions 22 to make the support rings 32movable along the axial directions of the trunnions 22 as well ascapable of tilting movements with respect to the trunnions 22. Inaddition, the support rings 32 and the rollers 34 are unitized via theneedle rollers 36 so as to be capable of relative rotations as describedabove. Therefore, the support rings 32 and rollers 34 are capable ofunitary tilting movements with respect to the trunnions 22. Here, theterm “tilt movements” refers to tilts the axes of the support rings 32and rollers 34 make with respect to the axes of the trunnions 22, withinthe planes containing the axes of the trunnions 22 (see FIG. 9(A)).

[0149] The support rings 32 may have an inner periphery shapedcylindrical over most of its width, whereas the generatrix to the innerperipheries of the support rings 32 here combines a cylindrical portion32 a at the center and relief portions 32 b on both sides. The role ofthe relief portions 32 b is to avoid the interference with the trunnions22 at an operating angle of θ as shown in FIGS. 9(A) and 11(A). Eachrelief portion 32 b is formed by a straight or curved line thatgradually spreads out in diameter from an edge of the arc portion 32 ato an end of the support ring 32. The relief portions 32 b illustratedhere are formed as part of a conical surface having a vertex angleα=50°.

[0150] In tripod type constant velocity universal joints, one rotationof the outer joint member 10 constitutionally produces three nutationsof the tripod member 20 with respect to the center of the outer jointmember 10. Here, the amount of eccentricity represented by the referencesymbol e (FIG. 9(A)) increases in proportion to the operating angle θ.While the three trunnions 22 are spaced by 120° from one another, theoperating angle θ causes the trunnions 22 to tilt as shown in FIG. 9(B).More specifically, with reference to the vertical trunnion 22 shown tothe upper in the diagram, the remaining two trunnions 22 are declinedslightly from their zero-operation-angle axes shown by the dot-dashlines. For example, an operating angle θ of approximately 23° causes adecline of the order of 2-3°. This decline can be readily allowed by thecurvature of the arc portions 32 a on the inner peripheries of thesupport rings 32 so that the surface pressures at the contact portionsbetween the trunnions 22 and the support rings 32 are prevented fromincreasing excessively.

[0151] In conventional joints, trunnions make contact with the innerperipheries of support rings at the full lengths of their outerperipheries, thereby creating circumferentially extended contactingellipses. Therefore, when the trunnions tilt with respect to the outerjoint member, there occur friction moments which function to tilt thesupport rings, and finally the rollers, with the movement of thetrunnions. On the other hand, in the embodiment shown in FIGS. 8(A) and8(B), the elliptic cross sections of the trunnions 22 and thecylindrical inner peripheries of the support rings 32 create contactingellipses closer to points as shown by the broken line in FIG. 10, with areduction in area at the same time. As a result, the forces to tilt theroller assemblies (32, 34) decrease greatly as compared to theconventional ones, with a further improvement in the orientationstability of the rollers 34.

[0152] Moreover, conventional joints have ribs for restraining theirrollers from tilting. These ribs are formed on the bottom sides of thetrack grooves, i.e., on the sides of greater diameter as seen in thecross section of the outer joint member 10, so as to be opposed to theend faces of the rollers. In the individual embodiments above as well asthose to be described later, however, roller-tilting factors arereduced. Accordingly, such ribs in the track grooves 12 are not alwaysrequired, and may be omitted. This eliminates the fear that the rollersmight come into contact with the ribs to produce sliding frictions whenthey are momentarily swung by some reason.

[0153]FIG. 12(A) through 14 show an embodiment in which balls 36′ areused as the rolling elements instead of the needle rollers 36 in theabove-described embodiments. There is no other essential differencesfrom what has been described in conjunction with FIGS. 8(A) through11(B), except in the following two points. First, the use of the balls36′ involves the formation of raceway surfaces in the outer peripheriesof the support rings 32 and the inner peripheries of the rollers 34.Second, the washers 33, 35 are eliminated, and support rings 32′ androllers 34′ are then provided with insert holes 33′ and 35′ for use inthe installation of the balls 36′.

[0154] In the constant velocity universal joint of the presentembodiment, the trunnions 22 having an elliptic cross section and thesupport rings 32 having a circular cross section make contact with eachother for torque transmission, as shown in FIGS. 11(A) and 11(B). It isthus desirable that the surface pressures therebetween be relaxed.Hereinafter, description will be given of the specific means for thatpurpose. Incidentally, the vertical direction in FIG. 11(B) representsthe load direction, and the horizontal the non-load direction.

[0155] When the joint transmits torque with an operating angle of θ,each trunnion 22 makes reciprocating movements with respect to thecorresponding support ring 32 within the bounds of the operating angleθ, as shown by the double-dashed chain lines in FIGS. 11(A) and 11(B).Here, in the non-load direction, the trunnion 22 and the support ring 32have a relatively large clearance, which allows the trunnion 22 to swingwithout interfering with the support ring 32. In the load direction,however, the trunnion 22 increases in apparent curvature as shown by thedouble-dashed chain line in FIG. 11(B) as the operating angle θ widensto increase the tilt of the trunnion 22. When the apparent curvatureexceeds the curvature of the inner diameter of the support ring 32, thetrunnion 22 comes into two-point contact with the support ring 32. Then,the trunnion 22 cannot tilt freely by itself any longer, and starts toinvolve the support ring 32, and finally the roller assembly (32, 34),in its inclination. On this account, the cross-sectional configurationof the trunnions 22, especially the dimensions in the load direction,are determined so that the trunnions 22 can swing within a predeterminedangle range by themselves without interfering with the support rings 32.

[0156] Specifically, assuming that the maximum operating angle θ max is25°, the following setting allows the joint to take the maximumoperating angle without tilting the support rings 32, and render thecontacting ellipses between the trunnions 22 and the support rings 32closer to a circle (minimum ellipse) at an operating angle of 0°:

[0157] b/a=0.841

[0158] R=2.380a,

[0159] where a and b are the semimajor and semiminor axes of thegenerally elliptic cross section of a trunnion 22, respectively, and Ris the radius of curvature of the inner periphery of a support ring, asshown in FIG. 15. Given that the radius of curvature R has arecommendable range of 0.5R and 1.5R, i.e., 1.190a and 3.570a, theellipticity b/a falls within the range of 0.983 and 0.669.

[0160] Although feasible in terms of configuration, the above-describedsetting may cause too high a surface pressure between the trunnions 22and the support rings 32 to make practical use of the joint for motorvehicles. Therefore, in the cases where vibrations are required underthe range of normal operating angles in automotive applications, theoperating angle can be lowered to the extent that the roller assemblies(32, 34) will not tilt. This decreases the surface pressures to allowpractical use of the joint. For example, Table 3 lists optimum valuesand recommendable ranges for the radius of curvature R and theellipticity b/a of the support ring's inner periphery, assuming that thenormal operating angle θ is greater than 100 and smaller than 20°.OPTIMUM VALUE OPERATING (MINIMUM ANGLE θ SURFACE RECOMMENDABLE RANGE (°)PRESSURE) 0.5 r 1.5 r R 20 2.939a 1.469a 4.408a 15 3.888a 1.944a 5.832a10 5.810a 2.905a 8.715a b/a 20 0.866 0.972 0.745 15 0.894 0.968 0.812 100.925 0.973 0.875

[0161] As mentioned previously, the smaller the ellipticity b/a of theelliptic cross sections of the trunnions 22 is, the greater operatingangle the trunnions 22 can take without tilting the roller assemblies(32, 34). Smaller ellipticities, however, increases the surfacepressures on the contact areas and decreases the strength of thetrunnions 22. Accordingly, in the embodiment shown in FIG. 16, atrunnion 22 is provided with a cross section of composite ellipticconfiguration. That is, a greater ellipticity b1/a1 is appliedexclusively to the areas for making contact with a support ring 32,i.e., to the contact areas β, while the remaining non-contact areas areformed with an ellipticity b2/a2 such as simply prevents theinterference at the maximum operating angle. For example, given that thenormal operating angle θ max is 15° and the radius of curvature R forthe inner periphery of the support ring 32 is 3.888a, the ellipticityb1/a1 to the contact areas and the ellipticity b2/a2 to the non-contactareas are set at 0.894 and 0.704, respectively. Incidentally, FIG. 16shows only one contact area β to the lower. It seems needless to addthat another contact area exists to the upper in the diagram since thetrunnion 22 has a symmetric cross section.

[0162] Moreover, FIG. 17 shows an embodiment in which the contact areasβ mentioned above are not made of a single ellipse but formed with acontinuously varying ellipticity (b/a). For example, on the sameassumption as employed above that the normal operating angle θ max is15° and the radius of curvature R to the inner periphery of the trunnion22 is 3.888a, the ellipticity varies as follows: That is, in the contactareas, the ellipticity starts with a value of 1.0 at the intersectionswith the major axis. It gradually decreases with increasing distancefrom the intersections. Then, the ellipticity ends up with a value of0.704 in the non-contact areas. Alternatively, the ellipticity maydecrease gradually from 1.0 to 0.704 as approaching from the major-axissides to the minor-axis sides, irrespective of contact and non-contactareas. Shown in FIG. 17 is an example in which the ellipticity is 1.0 atthe intersections of the contact areas with the major axis, and theradius of curvature gradually decreases with increasing distance fromthe intersections, for example, at predetermined angles as shown in thediagram.

[0163] Since the trunnion 22 has an elliptic cross section as describedabove, grinding has only to be applied to the load-side contact areas(β) where a high degree of precision is needed. The remainingnon-contact areas may be machined so as to retreat to smaller diametersthan the diameter of the original ellipse (shown by the double-dashedchain line in FIG. 17) for the purpose of grind relief. In thisconnection, the formation of the grind relief portions does notnecessarily require intentional application of cutting or othermachining. These portions may be so shaped upon the forging of thetrunnions and then left forge-finished. This reduces the machining timeand cuts down the cost as well.

[0164] FIGS. 18(A) through 19(B) show a tripod type constant velocityuniversal joint according to a fourth embodiment of the presentinvention. FIG. 18(A) shows a cross section of the joint, FIG. 18(B) asection perpendicular to a trunnion, and FIG. 18(C) a section of asupport ring. FIG. 19(A) shows a longitudinal section of the joint withan operating angle (θ).

[0165] As shown in FIGS. 18(A)-18(C), the constant velocity universaljoint is chiefly composed of an outer joint member 10 and a tripodmember 20. The outer joint member 10 is connected to one of two shaftsto be coupled, and the tripod member 20 is connected to the other.

[0166] The outer joint member 10 has three track grooves 12 axiallyextending in its inner periphery. Each of the track grooves 12 hasroller guideways 14 formed on its circumferentially-opposed side walls.The tripod member 20 has three trunnions 22 projecting radially. Aroller 34 is attached to each of the trunnions 22, and this roller 34 isaccommodated in one of the track grooves 12 in the outer joint member10. The roller 34 has an outer periphery that forms a convex surfaceconforming to the roller guideways 14.

[0167] Here, the outer periphery 34 a of each roller 34 forms a convexsurface whose generatrix is an arc having the center of curvatureradially off the axis of the trunnion 22. The roller guideways 14 have asection of Gothic arc shape. Thus, the outer peripheries 34 a of therollers 34 and the roller guideways 14 make angular contacts with eachother. In FIG. 18(A), dot-dash lines show the two contact positions. Thespherical outer peripheries of the rollers may also be combined withtapered cross sections of the roller guideways 14 to achieve angularcontacts therebetween. The adoption of such constitutions as provideangular contacts between the rollers 34 and the roller guideways 14makes the rollers less prone to vibrate, thereby stabilizing theorientations of the rollers. Incidentally, when angular contacts are notadopted, the roller guideways 14 may be formed, for example, by part ofa cylindrical surface whose axis is parallel to that of the outer jointmember 10. In this case, the cross-sectional configuration of theguideways 14 may be an arc corresponding to the generatrix to the outerperipheries of the rollers 34.

[0168] A support ring 32 is fitted onto the outer periphery 22 a of eachtrunnion 22. This support ring 32 and the roller 34 are unitized via aplurality of needle rollers 36 to constitute a roller assembly capableof relative rotations. More specifically, the needle rollers 36 arerotatably interposed between inner and outer raceway surfaces, with thecylindrical outer periphery of the support ring 32 and the cylindricalinner periphery of the roller 34 as the inner and outer racewaysurfaces, respectively. As shown in FIG. 18(B), the needle rollers 36are arranged in a so-called full complement state, where the rollers areloaded in as many as possible without any retainer. The referencenumerals 33 and 35 represent a pair of engaging rings which are fittedto annular grooves formed in the inner periphery of each roller 34, withan aim to stop the needle rollers 36 from coming off.

[0169] In longitudinal section (FIG. 18(A)), the outer periphery 22 a ofeach trunnion 22 has a straight shape parallel to the axis of thetrunnions 22. In cross section (FIG. 18(B)), the trunnion 22 has theform of an ellipse whose major axis is orthogonal to the axis of thejoint. The cross section of the trunnion 22 is shaped to be generallyelliptic with a reduction in thickness as seen in the axial direction ofthe tripod member 20. In other words, each trunnion has such across-sectional configuration that the faces opposed to each other inthe axial direction of the tripod member retreat toward each other,i.e., to smaller diameters than the diameter of the imaginarycylindrical surface.

[0170] The inner periphery of each support ring 32 is arcuate and convexin section. That is, the generatrix to the inner periphery 32 c is aconvex arc having a radius of r (FIG. 18(C)). This combines with theabove-described general elliptic cross sections of the trunnions 22 andthe provision of predetermined clearances between the trunnions 22 andthe support rings 32, to make the support rings 32 movable along theaxial directions of the trunnions 22 as well as capable of tiltingmovements with respect to the trunnions 22. In addition, the supportrings 32 and the rollers 34 are assembled (unitized) via the needlerollers 36 to be capable of relative rotations, as described above.Therefore, the support rings 32 and rollers 34 are capable of unitarytilting movements with respect to the trunnions 22. Here, the term “tiltmovements” refers to the tilts the axes of the support rings 32 androllers 34 make with respect to the axes of the trunnions 22, within theplanes containing the axes of the trunnions 22 (see FIG. 19(A)).

[0171] In conventional joints of this kind, trunnion make contact withthe inner peripheries of support rings at the full lengths of theirouter peripheries. This produces circumferentially extended contactingellipses. Therefore, when the trunnions tilt with respect to the outerjoint member, there arise friction moments which function to tilt thesupport rings, and finally the rollers, with the movement of thetrunnions. On the other hand, in the embodiment shown in FIGS.18(A)-18(C), the generally elliptic cross sections of the trunnions 22and the cylindrical cross sections of the inner peripheries of thesupport rings 32 make contacting ellipses closer to points as shown bythe broken line in FIG. 18(C), with a reduction in area at the sametime. As a result, the forces to tilt the roller assemblies (32, 34, 36)decrease greatly as compared to the conventional ones, with a furtherimprovement in the orientation stability of the rollers 34.

[0172] In the above-described constitutions, the tripod member 20 ismade of steel material having a carbon content of 0.15-0.40% by weight,through the major processes of forging→machining→carburizing andtempering→grinding of the outer peripheries 22 a of the trunnions 22.The softening resistance characteristic value R on the outer peripheries22 a of the trunnions 22 and other surfaces of the completed tripodmember 20 is limited to the range of 705<R≦820, and preferably710<R≦810. Accordingly, in the tripod member 20, the outer peripheries22 a of the trunnions 22 have a longer rolling fatigue life. Moreover,the bottom portions of the trunnions 22 and the serration portion (orspline portion) have higher torsional fatigue strength and the like aswell as excellent durability and strength.

[0173] In this connection, adoption of carbonitriding and temperinginstead of the carburizing and tempering in the processes describedabove is more effective in enhancing the rolling fatigue life, torsionalfatigue life, and the like.

[0174] Moreover, the surface layers formed by the carburizing andtempering (carburized layers) or the surface portions formed by thecarbonitriding and tempering (carbonitrided layers) can be adjusted to20-40% by volume in residual austenite content to improve the surfacecrack sensitivity for yet longer rolling fatigue life.

[0175] Otherwise, the tripod member 20 may be made of steel materialhaving a carbon content of 0.45-0.60% by weight, through the majorprocesses of forging→machining→induction hardening andtempering→grinding of the outer peripheries 22 a of the trunnions 22.Here, the softening resistance characteristic value R on the outerperipheries 22 a of the trunnions 22 and other surfaces of the completedtripod member 20 is limited to the range of 630<R>820, and preferably640<R≦810. Accordingly, in the tripod member 20, the outer peripheries22 a of the trunnions 22 have a longer rolling fatigue life. Besides,the bottom portions of the trunnions 22 and the serration portion (orspline portion) have higher torsional fatigue strength and the like aswell as excellent durability and strength. Incidentally, the inductionhardening and tempering may be applied to all over the outer peripheries22 a and the bottom portions of the trunnions 22, or be locally appliedto only the vicinities of certain points on a plane that includes theaxes of the trunnions 22 and intersects the axis of the tripod member 20at right angles. The carburizing/tempering and thecarbonitriding/tempering become also feasible for such localapplications when accompanied by anti-carburizing and -nitridingtreatments.

[0176] The outer joint member 10 is made of steel material having acarbon content of 0.15-0.40% by weight, through the major processes offorging→machining→carburizing and tempering→grinding of a shaft portion10 a (see FIG. 19(A)). The carburizing and tempering may be replacedwith carbonitriding and tempering, or induction hardening and tempering.Since the limitation of the softening resistance characteristic value Rand the other respects are in conformity to those of the tripod member20, repetitive description thereof will be omitted.

[0177] Additionally, the outer peripheries 22 a of the trunnions 22 ofthe tripod member 20 and the roller guideways 14 in the outer jointmember 10 may be provided with the above-described minute dimples and/orsolid lubrication coatings having chemical conversion undercoatings.Cold sulfurizing is also applicable.

[0178] After the above-described major processes are completed, shotpeening may be applied to at least one portion among the outerperipheries 22 a of the trunnions 22, the bottom portions of the same,and the serration portion (or spline portion) of the tripod member 20,and to at least either the roller guideways 14 or the shaft portion 10 a(the serration portion or spline portion, in particular) of the outerjoint member 10. The application of shot peening produces finer surfacestructures, and causes residual compressive stress on the surfaces. Thismeans improved rolling fatigue life and enhanced strengths againsttorsional fatigue and the like. Besides, in the cases where carburizedlayers or carbonitrided layers are formed, high impact energy from theshot particles causes martensite transformation of the residualaustenite in the surface portions. This further increases the residualcompressive stress as well as provides the surfaces with minute dimplesto make oil sumps, yet effectively improving the wear resistance andenhancing the rolling fatigue life and torsional fatigue strength. Thistendency is particularly significant in carbonitrided layers which arehigh in residual austenite content.

[0179] In the constant velocity universal joint of the presentembodiment, the tripod member 20 and the outer joint member 10 areoptimized in material, surface, and subsurface properties, as well asimproved in rolling fatigue life and in the strengths against torsionalfatigue and the like. As a result, this constant velocity universaljoint has superior durability and strengths as compared with existingconstant velocity universal joints of equivalent sizes. Besides, morecompact configuration is available while securing durability andstrengths equivalent to or higher than those of existing products.

[0180] FIGS. 20(A) through 21 show a tripod type constant velocityuniversal joint according to a fifth embodiment of the presentinvention. This fifth embodiment differs from the fourth embodimentdescribed above only in that the generatrix to the inner peripheries 32c of the support rings 32, which has been a single arc in the fourthembodiment, consists of a combination of an arc portion 32 a at thecenter and relief portion 32 b on both sides. The role of the reliefportions 32 b is to avoid the interference with trunnions 22 at anoperating angle (θ) as shown in FIG. 20(C). Each relief portion 32 b isformed by a straight or curved line that gradually spreads out indiameter from an edge of the arc portion 32 a to an end of the supportring 32. The relief portions 32 b illustrated here are formed by part ofa conical surface having a vertex angle α=50. The arc portion 32 a has alarge radius of curvature (r), for example, of the order of 30 mm sothat the trunnion 22 can make a tilt of 2-3° or so with respect to thesupport ring 32.

[0181] In tripod type constant velocity universal joints, one rotationof the outer joint member 10 constitutionally produces three nutationsof the tripod member 20 with respect to the center of the outer jointmember 10. Here, the amount of eccentricity represented by the referencesymbol e (FIG. 19(A)) increases in proportion to the operating angle(θ). While the three trunnions 22 are spaced by 120° from one another,the operating angle (θ) causes the trunnions 22 to tilt as shown in FIG.19(B). More specifically, with reference to the vertical trunnion 22shown to the upper in the diagram, the remaining two trunnions 22 aredeclined slightly from their zero-operation-angle axes shown by thedot-dash lines. For example, an operating angle (θ) of approximately 23°causes a decline of the order of 2-3°. This decline can be readilyallowed by the curvature of the arc portions 32 a on the innerperipheries of the support rings 32. Therefore, the surface pressures atthe contact portions between the trunnions 22 and the support rings 32can be prevented from increasing excessively. Here, FIG. 19(B) is aschematic representation of the three trunnions 22 of the tripod member20 as seen from the left side of FIG. 19(A), the trunnions beingrepresented by the full lines.

[0182] In the present embodiment, the tripod member 20 and the outerjoint member 10 are also optimized in material, surface, and subsurfaceproperties, as well as improved in rolling fatigue life and in thestrengths against torsional fatigue and the like. As a result, theconstant velocity universal joint of the present embodiment has superiordurability and strengths as compared with existing constant velocityuniversal joints of equivalent sizes. Besides, more compactconfiguration is available while securing durability and strengthsequivalent to or higher than those of existing products.

[0183] FIGS. 22(A) through 23 show a tripod type constant velocityuniversal joint according to a sixth embodiment of the presentinvention. Here, FIGS. 22(A) and 22(B) show the joint at an operatingangle of 0°, under no rotational torque.

[0184] The tripod type constant velocity universal joint of the presentembodiment comprises an outer joint member 1 to be connected to one oftwo shafts to be coupled, and a tripod member 2 to be connected to theother.

[0185] The outer joint member 1 is generally cup-like in appearance, andhas an inner periphery provided with three axially-extending trackgrooves 1 a at circumferential regular positions. Each of the trackgrooves 1 a has roller guideways 1 a 1 on both sides.

[0186] The tripod member 2 has three radially-projecting trunnions 2 aat circumferential regular positions. Each of the trunnions 2 a has aconvex-arcuate outer periphery 2 a 1. Onto the outer periphery 2 a 1 ismounted a roller assembly A consisting of a support ring 3, a pluralityof needle rollers 4, and a roller 5.

[0187] As magnified in FIG. 22(B), each roller assembly A includes theplurality of needle rollers 4 rotatably interposed between a cylindricalouter periphery 3 a of the support ring 3 and a cylindrical innerperiphery 5 a of the roller 5. A pair of snap rings 6 fitted to theinner periphery 5 a of the roller 5 engaging the support ring 3 and theneedle rollers 4 at both ends so as to restrain axial movements of thesupport ring 3 and the needle rollers 4 with respect to the roller 5(movements along the axis Z of the trunnion 2 a). The end faces of thesupport ring 3 and the end faces of the needle rollers 4 have axialclearances δ from the pair of snap rings 6. In the diagram, the axialclearances δ are rather exaggerated in dimension. The axial clearance δbetween the end faces of the support ring 3 and the snap rings 6 and theaxial clearance δ between the end faces of the needle rollers 4 and thesnap rings 6 can be designed in an identical value or in differentvalues. In the diagrams, both the clearances are shown as an axialclearance δ without distinction. Furthermore, the outer periphery 3 a ofthe support ring 3 and the inner periphery 5 a of the roller 5 haveslight radial clearances from the rolling contact surfaces of the needlerollers 4.

[0188] The inner peripheries 3 b of the support rings 3 are fitted tothe spherical outer peripheries 2 a 1 of the trunnions 2 a. In thisembodiment, the inner periphery 3 b of each support ring 3 has the formof a cone gradually contracting in diameter toward the extremity of thetrunnion 2 a, and makes line contact with the outer periphery 2 a 1 ofthe trunnion 2 a. This permits tilting movements of the rollerassemblies A with respect to the trunnions 2 a. The inner peripheries 3b of the support rings 3 have an inclination a as small as 0.1-3° forexample, and preferably 0.1-1°. The present embodiment employs thesetting of α=0.5°. In the diagrams, the inclinations of the innerperipheries 3 b are rather exaggerated.

[0189] The generatrix to the outer periphery 3 b of each roller 5 is anarc whose center is outwardly off the center of the trunnion 2 a.

[0190] In the present embodiment, the roller guideways 1 a 1 in theouter joint member 1 have a section of double-arc shape (Gothic archshape). Therefore, the roller guideways 1 a 1 and the outer periphery 5b of each roller 5 make angular contact at two points p and q. Theangular contact points p and q are opposed to each other in thedirection of the axis Z of the trunnion 2 a, at equal distance from acenter line that passes through the center of the outer periphery 5 b ofthe roller 5 and intersects the axis Z at right angles. Incidentally,the roller guideways 1 a 1 may have a section of V shape, parabolashape, or the like. Additionally, in the present embodiment, shouldersurfaces 1 a 2 are arranged next to the roller guideways 1 a 1 so thatan end face 5 c of the roller 5 is guided by these shoulder surfaces 1 a2.

[0191] Since the inner periphery 3 b of each support ring 3 is shapedlike a cone that gradually contracts in diameter toward the trunnionextremity, the application of rotational torque to this joint produces aload component F as shown in FIG. 23 (where the inclination of the innerperiphery 3 b is exaggerated more than in FIGS. 22(A) and 22(B)). Morespecifically, a load component F directed to the trunnion extremityoccurs at the contact position S between the inner periphery 3 b of thesupport ring 3 and the outer periphery 2 a 1 of the trunnion 2 a. Thisload component F acts to push up the support ring 3 and the needlerollers 4 toward the trunnion extremity so that the support ring 3 andthe needle rollers 4 are pressed against the snap ring 6 on thetrunnion-extremity side. This stabilizes the contact position S betweenthe inner periphery 3 b of the support ring 3 and the outer periphery 2a 1 of the trunnion 2 a. Besides, the load component F also acts to pushup the roller 5 toward the trunnion extremity through the medium of thesupport ring 3 and the needle rollers 4, thereby stabilizing theorientation of the roller 5. Such stabilization of the contact positionS and the orientation stabilization of the roller 5 combine with eachother to reduce and stabilize the induced thrust effectively.Incidentally, the inner periphery 3 b of the support ring 3 may have acylindrical configuration.

[0192] As in the embodiments described previously, the tripod member 2and the outer joint member 1 are optimized in material, surface, andsubsurface properties, as well as improved in rolling fatigue life andin the strengths against torsional fatigue and the like. As a result,the constant velocity universal joint of the present embodiment hassuperior durability and strengths as compared with existing constantvelocity universal joints of equivalent sizes. Besides, more compactconfiguration is available while securing durability and strengthsequivalent to or higher than those of existing products.

[0193] FIGS. 24(A) through 24(C) show a seventh embodiment of thepresent invention. Here, FIGS. 24(A)-24(C) show the joint at anoperating angle of 0°.

[0194] As shown in FIGS. 24(A)-24(C), the tripod type constant velocityuniversal joint of the present embodiment comprises an outer jointmember 1′ to be connected to one of two shafts to be coupled, and atripod member 2′ to be connected to the other. The outer joint member 1′is generally cup-like in appearance, and has an inner periphery providedwith three axially-extending track grooves 1 a′ at circumferentialregular positions. Each of the track grooves 1 a′ has roller guideways 1a′1 on both sides. The tripod member 2′ has three radially-projectingtrunnions 2 a′ at circumferential regular positions. Each of thetrunnions 2 a′ has a cylindrical outer periphery, on which an innerroller 3′ is rotatably mounted via a plurality of needle rollers 7′. Inaddition, an outer roller 4′ is rotatably fitted to outside of the innerroller 3′.

[0195] As magnified in FIG. 24(B), the needle rollers 7′ and the innerroller 3′ are engaginged at one ends by a stopper support ring 8′ and asnap ring 9′ both attached to the extremity of a trunnion 2 a′, andretained at the other ends by a washer 10′ attached to the bottom of thetrunnion 2 a′. Thereby, the needle rollers 7′ and the inner roller 3′are restrained from movements along the axis Z of the trunnion 2 a′. Infact, the needle rollers 7′ and the inner roller 3′ have slight axialclearances 6′ from the stopper support ring 8′ and the washer 10′. Inthe diagrams, the axial clearances δ′ are rather exaggerated in size.The outer periphery of the trunnion 2 a′ and the inner periphery 3′ ofthe inner roller 3 a′ also have slight radial clearances from the needlerollers 7′. The inner roller 3′ is cylindrical at the inner periphery 3a′ and convex arcuate at the outer periphery 3 b′. In the presentembodiment, the generatrix to the outer periphery 3 b′ is an arc havinga radius of r1, around a point O1′ which is outwardly off the radiuscenter O2′ of the inner roller 3′ by a predetermined distance. Theradius r1 is smaller than the maximum radius r2 of the outer periphery 3b′.

[0196] The outer roller 4′ is fitted to the outer periphery 3 b′ of theinner roller 3′. In the present embodiment, the inner periphery 4 a′ ofthe outer roller 4′ has the form of a cone gradually contacting indiameter toward the extremity of the trunnion 2 a′, and makes linecontact with the outer periphery 3 b′ of the inner roller 3′. Thispermits tilting movements of the outer roller 4′ with respect to thetrunnion 2 a′. The inner periphery 4 a′ has an inclination as small as0.1-3°, for example. The present embodiment employs the setting of0.3-0.7°. In the diagrams, the inclination of the inner periphery 4 a′is rather exaggerated. The generatrix to the outer periphery 4 b′ of theouter roller 4′ is an arc having a radius of r3 around a point O3′ stilloutward from the point O1′.

[0197] In the present embodiment, the roller guideways 1 a′1 of theouter joint member 1′ have a section of double-arc shape (Gothic archshape). Therefore, the roller guideways 1 a′1 and the outer periphery 4b′ of each outer roller 4′ make angular contact at two points p′ and q′.The angular contact points p′ and q′ are opposed to each other in thedirection of the axis Z of the trunnion 2 a′, at equal distances from acenter line that passes through the center O3′ of the outer periphery 4b′ of the outer roller 4′ and intersects the axis Z at right angles.Incidentally, the roller guideways 1 a′1 may have a section of V shape,parabola shape, or the like.

[0198] Since it is shaped like a cone that gradually contracts indiameter toward the trunnion extremity, the inner periphery 4 a′ of theouter roller 4′ produces a load component F directed to the trunnionextremity at its contact position S′ with the inner periphery 3 b′ ofthe inner roller 3′ as shown in FIG. 24(C). This load component F actsto push the outer roller 4′ toward the trunnion extremity, therebylowering the contact surface pressure at B portion of the rollerguideway 1 a′1 on the non-load side. As the reaction force to the loadcomponent F, a force directed to the trunnion bottom (downside in thediagram) also arises at the contact portion S′. This reaction force actsto push down the inner roller 3′ toward the trunnion bottom, therebyrestraining axial movements of the inner roller 3′ and the needlerollers 7′ with respect to the trunnion 2 a′. Consequently, as shown inFIG. 24(B), the inner roller 3′ and the needle rollers 7′ are pressedagainst the washer 10′ on the bottom side, whereby fluctuations of thecontact position S′ due to the axial clearances δ′ are suppressed. Suchreduction of contact surface pressure at B portion of the rollerguideway 1 a′1 on the non-load side and the stabilization of the contactposition S′ combine with each other to reduce and stabilize the inducedthrust effectively. Incidentally, the inner periphery 4 a′ of the outerroller 4′ may have a cylindrical configuration.

[0199] As in the embodiments described previously, the tripod member 2′and the outer joint member 1′ are optimized in material, surface, andsubsurface properties, as well as improved in rolling fatigue life andin the strengths against torsional fatigue and the like. As a result,the constant velocity universal joint of the present embodiment hassuperior durability and strengths as compared with existing constantvelocity universal joints of equivalent sizes. Besides, more compactconfiguration is available while securing durability and strengthsequivalent to or higher than those of existing products.

[0200] Note that the above-mentioned improvements through theoptimizations in the material, surface, and subsurface properties of thecomponent parts are not limited to constant velocity universal jointshaving the constitutions of FIGS. 18(A)-24(C), and may also be appliedto constant velocity universal joints having the constitutions of FIGS.1(A)-17. In addition, the improvements are also applicable to theconstant velocity universal joints comprising: roller guidewaysconsisting of flat surfaces; outer rollers having cylindrical outerperipheries and concave-spherical inner peripheries; and inner rollershaving convex-spherical outer peripheries, wherein slides between theconcave-spherical inner peripheries of the outer rollers and theconvex-spherical outer peripheries of the inner rollers permit thetilting movements of the outer rollers (Japanese Patent Application Nos.Hei 8-4073 and 8-138335). Likewise the constant velocity universaljoints in which the roller guideways and the axes of the trunnions areconfigured not to be parallel to each other at an operating angle of 0°(Japanese Patent Laid-Open Publication No. Hei 11-13779).

[0201] Tables 4 and 5 show the results of a test made on the tripodmember in the constant velocity universal joint shown in FIGS.18(A)-18(C). TABLE 4 SOFTENING RESISTANCE CHARACTER- CONTENTS OF MAJORISTIC COMPONENTS IN VALUE R SAMPLE STEEL MATERIAL (Wt %) (MEASUREMENT)No. C Si Mn Ni Cr Mo (Hv) 1 0.16 0.26 0.73 0 1.1 0 712 2 0.2 0.05 0.2 01 0 705 3 0.2 0.05 0.5 0 0.8 0 709 4 0.2 0.75 0.5 0 1.5 0 715 5 0.2 0.750.8 0.7 1.5 0 721 6 0.2 0.9 1 0.8 1 0 735 7 0.2 0.9 1.2 1 1.5 0 729 80.2 0.5 1.2 1.8 2 0.5 799 9 0.2 0.5 1 2 1 0.5 817 10 0.2 0.75 1.5 2 10.5 823 11 0.2 0.25 0.84 0 0.94 0.03 735 12 0.21 0.93 0.82 0 0.7 0 73013 0.21 0.09 0.84 0 1 0 688 14 0.23 0.02 0.65 0 1.13 0.49 770 15 0.230.03 0.95 0 1.14 0.3 774 16 0.23 0.04 0.81 0 1.14 0.41 774 17 0.4 0.240.8 0 1 0.24 750

[0202] TABLE 5 SAMPLE No. 2 3 6 15 8 9 10 SOFTENING MEASUREMENT 705 709735 774 799 817 823 RESISTANCE ESTIMATION 702 709 735 756 800 814 825CHARACTERISTIC VALUE R (Hv) DURABILITY Δ ◯ ◯ ◯ ⊚ ⊚ ⊚ FORGEABILITY ⊚ ⊚ ⊚◯ ◯ ◯ Δ

[0203] Initially, tripod members were fabricated by using steelmaterials having different contents of major components (samples Nos.1-17), and carburized at 950° C.×8 h, followed by tempering Of 200° C.×2h. Then, the outer peripheries of their trunnions were measured forsoftening resistance value R (the maximum Vickers hardness Hv within adepth of 0.5 mm from the outer periphery). The results are shown inTable 4. Incidentally, the outer peripheries of the trunnions weresubjected to grinding after the carburizing and tempering; therefore,the above-mentioned “depth of 0.5 mm” was based on the ground surfaces.Then, the samples were individually evaluated for durability andforgeability. Table 5 shows the relations of those evaluations to themeasurements and estimations (the estimations will be discussed later)of softening resistance characteristic value R (Hv) on six types of thesamples. In the evaluation fields, ⊚ represents full satisfaction of theintended property, ◯ satisfaction, and Δ dissatisfaction.

[0204] It is confirmed from the results shown in Table 5 that thecarburized and tempered articles offer satisfactory durability andforgeability when their softening resistance characteristic values R arelimited to the range of 705<R≦820, and preferably 710≦R≦815. Softeningresistance characteristic values R smaller than or equal to 705 provideunfavorable results in terms of durability, and those exceeding 820provide unfavorable results in terms of forgeability.

[0205] On the other hand, for the sake of fatigue strengths, the carboncontent of the base metal, which determines the hardness of coreportions, preferably ranges from 0.15% to 0.40% by weight. When thecarbon content of the base metal is below 0.15% by weight, thecarburizing requires longer time. At the same time, the core portionsfall short of hardness, failing to offer satisfactory fatigue strengths.In contrast, at carbon contents higher than 0.4% by weight, the coreportions become excessively high in hardness, with a considerabledecrease in toughness as well as an increase in distortion.

[0206] For the reasons stated above, when component parts such as thetripod member and the outer joint member are composed of carburized andtempered articles, it is desirable that these parts be made of steelhaving a carbon content of 0.15-0.40% by weight and be limited to therange of 705<R≦820, and preferably 710≦R≦815, in softeningcharacteristic value R. By so doing, the rolling fatigue life, thefatigue strengths, and the like can be enhanced to improve thedurability and secure the forgeability at the same time. Moreover, thelimitation of the softening resistance characteristic value R to theranges mentioned above improves the material in hardenability, therebyallowing deeper hardening than heretofore. This is yet effective inimproving the fatigue strengths and the like.

[0207] The above-described softening resistance characteristic values Rmay be determined from measurements, whereas they can be estimated withrelatively high accuracy by the following regression equation (a):

R(estimation)=713.4+{20.7×Si(%)}+{12.3×Mn(%)}+{6.4×Ni(%)}−{14.8×Cr(%)}+{159.0×Mo(%)}  (a)

[0208] The above regression equation (a) was obtained through multipleregression analyses on the softening resistance characteristic values R(measurements) of the 17 types of samples shown in Table 4 (samples Nos.1-17) and the contents of major components (wt %) in the respectivesamples. In this example, Si, Mn, Ni, Cr, and Mo are selected as themajor components. Carbon C is omitted from the variables since thecarburization uniformizes the samples in carbon content.

[0209] As shown in Table 5, the estimations of the softening resistancecharacteristic value R are closely analogous to the measurements.Therefore, limitation of this estimated value R to the range of705<R≦820, and preferably 710≦R≦815, allows easy and relatively accurateevaluations of durability and forgeability.

[0210] Note that carbonitriding and tempering may be applied to thecomponent parts including the tripod member and the outer joint member.In that case, the same effects as those described above can be obtainedby limiting the carbon content and the softening resistancecharacteristic value R (measurement or estimation) of the base metal asin the carburized and tempered articles. Moreover, in the carbonitridedand tempered articles, the surface layers (carbonitrided layers) areappropriately increased in residual austenite content and improved incrack sensitivity. Therefore, carbonitrided and tempered articles aremore effective in enhancing the rolling fatigue life. Besides, thetrunnion bottoms and the serration portion increase in surface hardnesswith improvements in torsional fatigue strength and the like.

[0211] In the cases of the carburizing/tempering and of thecarbonitriding/tempering, a variety of steel materials shown in Table 8may be used aside from the steel materials shown in Table 4. TABLE 8SOFTENING RESISTANCE CONTENTS OF CHARACTERISTIC MAJOR COMPONENTS (Wt %)VALUE STEEL CODE C Si Mn Ni Cr Mo (MEASUREMENT) (Hv) SCr415 0.15 0.250.725 0.00 1.050 0.000 712 SCr420 0.20 0.25 0.725 0.00 1.050 0.000 712SCr430 0.30 0.25 0.725 0.00 1.050 0.000 712 SCr435 0.35 0.25 0.725 0.001.050 0.000 712 SCr440 0.40 0.25 0.725 0.00 1.050 0.000 712 SCM415 0.150.25 0.725 0.00 1.050 0.225 748 SCM420 0.20 0.25 0.725 0.00 1.050 0.225748 SCM430 0.30 0.25 0.725 0.00 1.050 0.225 748 SCM435 0.35 0.25 0.7250.00 1.050 0.225 748 SCM440 0.40 0.25 0.725 0.00 1.050 0.225 748 SNCM2200.20 0.25 0.750 1.25 0.525 0.225 764 SNCM415 0.15 0.25 0.550 1.80 0.5250.225 765 SNCM420 0.20 0.25 0.550 1.80 0.525 0.225 765 SNCM815 0.15 0.250.450 4.25 0.850 0.225 774

[0212] Tables 6 and 7 show the results of another test made on thetripod member in the constant velocity universal joint shown in FIGS. 18(A)-18 (C). TABLE 6 SOFTENING RESISTANCE CONTENTS OF MAJORCHARACTERISTIC COMPONENTS IN STEEL MATERIAL VALUE R SAMPLE No. C Si MnNi Cr Mo (MEASUREMENT) (Hv) 1 0.54 0.19 0.86 0 0.15 0 682 2 0.47 0.210.76 0 0.16 0 640 3 0.47 0.1 0.75 0 0.15 0 630 4 0.48 0.21 0.88 0.290.16 0.31 715 5 0.53 0.22 0.86 0 0.16 0.3 724 6 0.48 0.2 0.88 0 0.15 0.3701 7 0.48 0.2 0.75 0 0.15 0.3 695 8 0.48 0.2 0.88 0 0.15 0.25 689 90.48 0.15 0.83 0 0.1 0.3 712 10 0.48 0.05 0.88 0 0.15 0.3 689 11 0.480.1 0.88 0 0.02 0.3 705 12 0.48 0.15 0.88 0 0.02 0.3 715 13 0.48 0.20.88 0 0.02 0.3 718 14 0.45 0.8 1.1 0 0.15 0 692 15 0.54 0.24 1 1.1 0.10.5 810 16 0.53 0.49 1 1.2 0.12 0.5 827 17 0.52 0.25 0.87 0 0.15 0.4 74018 0.59 0.23 0.77 0 0.1 0 714

[0213] TABLE 7 SAMPLE No. 3 2 1 12 17 15 16 SOFTENING MEASUREMENT 630640 682 715 740 810 827 RESISTANCE ESTIMATION 633 641 679 714 742 811827 CHARACTERISTIC VALUE R (Hv) DURABILITY Δ ◯ ◯ ◯ ◯ ⊚ ⊚ FORGEABILITY ⊚⊚ ◯ ◯ ◯ ◯ Δ

[0214] Initially, tripod members were fabricated by using steelmaterials having different contents of major components (samples Nos.1-18), and subjected to induction hardening at 10 kHz×170 kW×3 sec.,followed by tempering of 200° C.×2 h. Then, the outer peripheries oftheir trunnions were measured for softening resistance value R (themaximum Vickers hardness Hv within a depth of 0.5 mm from the outerperiphery). Table 6 shows the results. Incidentally, the outerperipheries of the trunnions were subjected to grinding after theinduction hardening and tempering; therefore, the above-mentioned “depthof 0.5 mm” was based on the ground surfaces. Then, the samples wereindividually evaluated for durability and forgeability. Table 7 showsthe relations of those evaluations to the measurements and estimations(the estimations will be discussed later) of softening resistancecharacteristic value R (Hv) on seven types of the samples. In theevaluation fields, ⊚ represents full satisfaction of the intendedproperty, ◯ satisfaction, and Δ dissatisfaction.

[0215] It is confirmed from the results shown in Table 7 that theinduction hardened and tempered articles offer satisfactory durabilityand forgeability when their softening resistance characteristic values Rare limited to the range of 630<R≦820, and preferably 640≦R≦810.Softening resistance characteristic values R smaller than or equal to630 provide unfavorable results in terms of durability, and thoseexceeding 820 provide unfavorable results in terms of forgeability.

[0216] Meanwhile, in order to obtain sufficient hardness by theinduction hardening, the base metal needs to have a carbon contentwithin the range of 0.45-0.60% by weight.

[0217] For the reasons stated above, when component parts such as thetripod member and the outer joint member are composed of inductionhardened and tempered articles, it is desirable that these parts be madeof steel having a carbon content of 0.45-0.60% by weight and be limitedto the range of 630<R≦820, and preferably 640≦R≦810, in softeningcharacteristic value R. By so doing, the rolling fatigue life, thefatigue strengths, and the like can be enhanced to improve thedurability and secure the forgeability at the same time. The inductionhardening and tempering also produce residual compressive stresses onthe surfaces. Therefore, the induction hardening and tempering are moreeffective in enhancing the rolling fatigue life and the fatiguestrengths.

[0218] The above-described softening resistance characteristic values Rmay be determined from measurements, whereas they can be estimated withrelatively high accuracy by the following regression equation (b):

R(estimation)=378.0+{516.2×C(%)}+{83.2×Si(%)}+{31.8×Mn(%)}+{29.1×Ni(%)}−{132.6×Cr(%)}+{167.9×Mo(%)}  (b)

[0219] The above regression equation (b) was obtained through multipleregression analyses on the softening resistance characteristic values R(measurements) of the 18 types of samples shown in Table 6 (samples Nos.1-18) and the contents of major components (wt %) in the respectivesamples. In this example, C, Si, Mn, Ni, Cr, and Mo are selected as themajor components.

[0220] As shown in Table 7, the estimations of the softening resistancecharacteristic value R are closely analogous to the measurements.Therefore, limitation of the estimations R to the range of 630<R≦820,and preferably 640≦R≦810, allows easy and relatively accurateevaluations of durability and forgeability.

[0221] In the cases of induction hardening and tempering, a variety ofsteel materials shown in Table 9 can be used aside from the steelmaterials shown in Table 6. TABLE 9 SOFTENING RESISTANCE CHARACTERISTICCONTENTS OF VALUE CODE MAJOR COMPONENTS (Wt %) (MEASUREMENT) STEEL C SiMn Ni Cr Mo (Hv) S45C 0.45 0.25 0.75 0 0.1 0 642 S48C 0.48 0.25 0.75 00.1 0 657 S50C 0.50 0.25 0.75 0 0.1 0 667 S53C 0.53 0.25 0.75 0 0.1 0683 S55C 0.55 0.25 0.75 0 0.1 0 693 S58C 0.58 0.25 0.75 0 0.1 0 709 S61C0.61 0.25 0.75 0 0.1 0 724

[0222] While the test results described above are of the tripod member,similar results were obtained from other component parts such as theouter joint member, the rollers, and the support rings. Furthermore,similar results were also obtained from the constant velocity universaljoints of the other embodiments.

[0223] Otherwise, in the constant velocity universal joint having theconstitution shown in FIGS. 18(A)-18(C), the tripod member 20 is made ofsteel material having a carbon content of 0.15-0.40% by weight, throughthe major processes of forging→machining→carbonitriding andtempering→grinding of the outer peripheries 22 a of the trunnions 22.Here, the carbonitriding and tempering form surface portions(carbonitrided layers) directly beneath the surfaces of the tripodmember 20. The surface layers are limited to the range of 20≦γR≦40 inresidual austenite content γR (vol %). Incidentally, the surface layers(carbonitrided layers) have only to be formed at least beneath the outerperipheries 22 a of the trunnions 22. In the present embodiment, theouter peripheries 22 a of the trunnions 22 and other surfaces of thecompleted tripod member 20 is limited to the range of 705<R≦820, andpreferably 710<R≦810, in softening resistance characteristic value R.

[0224] In this connection, the carbonitriding and tempering in theprocesses described above may be replaced with carburizing and temperingwhile the surface layers (carburized layers) formed by the carburizingand tempering are limited to the range of 20≦γR≦40 in residual austenitecontent γR (vol %).

[0225] The outer joint member 10 is made of steel material having acarbon content of 0.15-0.40% by weight, through the major processes offorging→machining→carbonitriding and tempering→grinding of the shaftportion 10 a. The carbonitriding and tempering may be replaced withcarburizing and tempering. Since the other respects are in conformity tothose of the tripod member 20, repetitive description thereof will beomitted here.

[0226] The support rings 32, the rollers 34, and the needle rollers 36which constitute the roller assemblies are made of steel material havinga carbon content of 0.95-1.10% by weight, such as SUJ2 and other bearingsteels, through the major processes of forging→machining→nitriding andtempering→grinding. Here, the nitriding and tempering create nitridelayers (layers having nitride solid solution) as surface portionsdirectly beneath the surfaces of these component parts. The surfaceportions are limited to the range of 20≦γR≦40 in residual austenitecontent γR (vol %). In other respects including the materials andfabrication processes, these component parts may conform to the tripodmember 20 and the outer joint member 10 described above.

[0227] Additionally, the contact surfaces of the tripod member 20, theouter joint member 10, the support rings 32, the rollers 34, and theneedle rollers 36 may be provided with the above-described minutedimples and/or solid lubrication coatings having chemical conversionundercoatings. Cold sulfurizing is also applicable.

[0228] After the above-described major processes are completed, shotpeening may also be applied to at least one portion among the outerperipheries 22 a of the trunnions 22, the bottom portions thereof, andthe serration portion (or spline portion) of the tripod member 20, andto at least either the roller guideways 14 or the shaft portion 10 a(the serration portion or spline portion, in particular) of the outerjoint member 10. The application of shot peening produces finer surfacestructures, and causes residual compressive stress on the surfaces. Thismeans improved rolling fatigue life and enhanced strengths againsttorsional fatigue and the like. Besides, high impact energy from theshot particles causes martensite transformation of the residualaustenite in the surface portions. This further increases the residualcompressive stress as well as makes minute dimples to form oil sumps,yet effectively improving the wear resistance and enhancing the rollingfatigue life and torsional fatigue strength. This tendency isparticularly significant in carbonitrided layers which are high inresidual austenite content.

[0229] In the constant velocity universal joint of the presentembodiment, the component parts are optimized in material and surfaceproperties, as well as improved in rolling fatigue life and in thestrengths against cracks and the like. As a result, this constantvelocity universal joint has superior durability and strengths ascompared with existing constant velocity universal joints of equivalentsizes. Besides, more compact configuration is available while securingdurability and strengths equivalent to or higher than those of existingproducts.

[0230] The above-mentioned improvements through the optimizations in thematerial and surface properties of the component parts are not limitedto constant velocity universal joints having the constitutions of FIGS.18(A)-24(C), and may also be applied to constant velocity universaljoints having the constitutions of FIGS. 1(A)-17. In addition, theimprovements are also applicable to the constant velocity universaljoints comprising: roller guideways consisting of flat surfaces; outerrollers having cylindrical outer peripheries and concave-spherical innerperipheries; and inner rollers having convex-spherical outerperipheries, wherein slides between the concave-spherical innerperipheries of the outer rollers and the convex-spherical outerperipheries of the inner rollers permit the tilting movements of theouter rollers (Japanese Patent Application Nos. Hei 8-4073 and8-138335). Likewise the constant velocity universal joints in which theroller guideways and the axes of the trunnions are configured not to beparallel to each other at an operating angle of 0° (Japanese PatentLaid-Open Publication No. Hei 11-13779).

[0231] Rolling fatigue life tests were made on constant velocityuniversal joints having the constitution shown in FIGS. 18(A)-18(C),with carbonitrided-and-tempered surface layers (carbonitrided layers)formed on their tripod members. Initially, a plurality of tripod memberswere fabricated for each of the types with surface layers of thefollowing residual austenite contents (vol %): below 20, 20, 22, 25, 28,30, 35, 40, and above 40 (samples Nos. 18-26). These tripod members werebuilt into constant velocity universal joints, and run under identicalconditions for power recirculation type endurance tests. Then, Weibullevaluations were made on each of the types, with the assumption that therunning time reaches the life when damages (exfoliations, abrasions, andthe like) to the outer peripheries of the trunnions exceed a certaindegree. The results are collectively shown in Table 10. In theevaluation fields, ⊚ represents full satisfaction of the intended time,◯ satisfaction, and Δ dissatisfaction. TABLE 10 SAMPLE No. 18 19 20 2122 23 24 25 26 RESIDUAL UNDER 20 22 25 28 30 35 40 OVER AUST- 20 40ENITE CONTENT γR (vol %) ROLLING Δ ◯ ◯ ⊚ ⊚ ⊚ ⊚ ◯ Δ FATIGUE LIFE

[0232] It is confirmed from the results shown in Table 10 that limitingthe residual austenite content of the surface layers to the range of 20%to 40% by volume provides favorable rolling fatigue life. In particular,the range of 25% to 35% by volume offers preferable results.

[0233] While the test results described above are of the tripod member,similar results were obtained from other parts such as thoseconstituting the roller assemblies (the rollers, the needle rollers).Furthermore, similar results were also obtained from the constantvelocity universal joints of the other embodiments. Description of thesetest results will be omitted here.

[0234] Tests were also made on the softening resistance characteristicvalue R, with the same results as shown in Tables 4 and 5. Since theparticulars about the softening resistance characteristic value R areidentical to those described previously in conjunction with Tables 4, 5,and 8, repetitive description thereof will be omitted.

[0235] Incidentally, when the parts constituting the roller assembliesare to be nitrided and tempered, they can use high carbon chrome steel.More specifically, a variety of bearing steels shown in Table 11 can beused.

[0236] Otherwise, in the constant velocity universal joint having theconstitution shown in FIGS. 18(A)-18(C), the tripod member 20 is made ofsteel for carburization, having a carbon content of 0.15-0.40% byweight, through the major processes of forging→machining→heavycarburizing and tempering→grinding of the outer peripheries 22 a of thetrunnions 22. FIG. 25 shows an example of process conditions for plasmacarburizing (in FIG. 25, the carburizing is performed, for example, inthe steps of 920° C.×3.5 h and 890° C.×1.5 h). The heavy carburizing andtempering form surface layers (carburized layers) directly beneath thesurfaces of the tripod member 20. The surface portions contain astructure in which carbide is distributed into a martensite matrix.Alternatively, the tripod member 20 may be made of bearing steel, suchas SUJ2, and subjected to dip quenching and tempering. In this case, theprocess conditions may be as follows: 840° C.×30 minutes (heating)→110°C. (oil quenching)→180° C.×100 minutes (tempering). Since the outerperipheries 22 a of the trunnions 22 are sometimes ground toconsiderable depths, the latter constitution with deeper hardening iseffective.

[0237] Here, the above-mentioned carbide consists mainly of Fe₃C, to bemore specific. The structure having such a carbide distributed into itsmartensite matrix can be formed by providing at least the surface layerswith carbon C as much as or more than its eutectic point (0.8% by weightor higher), and subjecting the same to hardening and tempering. Inparticular, spheroidizing can be performed in the forming process of theparts, or appropriate adjustments can be made to the component contentsof the steel material and to the heat treatment conditions so that theabove-mentioned carbide car spheroidized for yet preferable results.

[0238] The outer joint member 10 is made of steel material having acarbon content of 0.15-0.40% by weight, through the major processes offorging→machining→carburizing and tempering→grinding of the shaftportion 10 a. The carburizing and tempering may be replaced withcarbonitriding and tempering.

[0239] The support rings 32, the rollers 34, and the needle rollers 36which constitute the roller assemblies are made of steel forcarburization, having a carbon content of 0.15-0.40% by weight, throughthe major processes of forging→machining→heavy carburizing andtempering→grinding. Here, the heavy carburizing and tempering createsurface layers (carburized layers) directly beneath the surfaces ofthese component parts. The surface layers contain the structure thatcarbide is distributed into a martensite matrix. Alternatively, thesecomponent parts may be formed of bearing steel, such as SUJ2, andsubjected to dip quenching and tempering. In other respects, thesecomponent parts are in conformity to the tripod member 20; therefore,description thereof will be omitted.

[0240] Additionally, the contact surfaces of the tripod member 20, theouter joint member 10, the support rings 32, the rollers 34, and theneedle rollers 36 may be provided with the above-described minutedimples and/or solid lubrication coatings having chemical conversionundercoatings. Cold sulfurizing is also applicable.

[0241] Furthermore, after the above-mentioned major processes arecompleted, shot peening may also be applied to at least one portionamong the outer peripheries 22 a of the trunnions 22, the bottomportions thereof, and the serration portion (or spline portion) of thetripod member 20, and to at least either the roller guideways 14 or theshaft portion 10 a (the serration portion or spline portion, inparticular) of the outer joint member 10. The application of shotpeening produces finer surface structures, and causes residualcompressive stress on the surfaces. This means improved rolling fatiguelife and enhanced strengths against torsional fatigue and the like.

[0242] In the constant velocity universal joint of the presentembodiment, the component parts are optimized in material and surfaceproperties, as well as improved in rolling fatigue life and in thestrengths against cracks and the like. As a result, this constantvelocity universal joint has superior durability and strengths ascompared with existing constant velocity universal joints of equivalentsizes. Besides, more compact configuration is available while securingdurability and strengths equivalent to or higher than those of existingproducts.

[0243] The above-described improvements through the optimizations in thematerial and surface properties of the component parts are not limitedto constant velocity universal joints having the constitution of FIGS.18(A)-18(C), and may also be applied to constant velocity universaljoints having the constitutions of FIGS. 20(A)-24(B) and to constantvelocity universal joints having the constitutions of FIGS. 1(A)-17. Inaddition, the improvements are also applicable to the constant velocityuniversal joints comprising: roller guideways consisting of flatsurfaces; outer rollers having cylindrical outer peripheries andconcave-spherical inner peripheries; and inner rollers havingconvex-spherical outer peripheries, wherein slides between theconcave-spherical inner peripheries of the outer rollers and theconvex-spherical outer peripheries of the inner rollers permit thetilting movements of the outer rollers (Japanese Patent Application Nos.Hei 8-4073 and 8-138335). Likewise the constant velocity universaljoints in which the roller guideways and the axes of the trunnions areconfigured not to be parallel to each other at an operating angle of 0°(Japanese Patent Laid-Open Publication No. Hei 11-13779).

[0244] The following tests were conducted to confirm the effect ofproviding the component parts' surface layers with the structure thatcarbide is distributed into a martensite matrix. The tests were made onembodiments and a comparative example, each of which was a constantvelocity universal joint having the constitution shown in FIGS.18(A)-18(C). The materials of the tripod members and the methods of heattreatment were as listed below.

[0245] Embodiment 1: with a tripod member of SCM420 steel, heavycarburized and tempered

[0246] Embodiment 2: with a tripod member of SUJ2 steel, complete dipquenching and tempering

[0247] Comparative example: with a tripod member of SCM420 steel,ordinarily carburized and tempered

[0248] (Test Conditions)

[0249] torque: 686 Nm, revolutions: 250 rpm, operating angle θ: 10degrees

[0250] test time: 300 h

[0251] After the tests under these conditions, the tripod members wereevaluated for rolling fatigue life on the outer peripheries of theirtrunnions. Table 12 shows the results. In the evaluation fields, ⊚represents full satisfaction of the intended time, ◯ satisfaction, and Δdissatisfaction. TABLE 12 ROLLING FATIGUE MATERIAL HEAT TREATMENT LIFEEMBODIMENT 1 SCM420 HEAVY ◯ CARBURIZING AND TEMPERING EMBODIMENT 2 SUJ2DIP QUENCHING ◯ AND TEMPERING EMBODIMENT 3 SCM420 CARBURIZING Δ ANDTEMPERING

[0252] It is confirmed from the results shown in Table 12 that theconstitutions of the embodiments 1 and 2, or the constitutions withcarbide distributed into a martensite matrix, provide satisfactoryrolling fatigue life.

[0253] While the test results described above are of the tripod member,similar results were obtained from other component parts such as theouter joint member. Furthermore, similar results were also obtained fromthe constant velocity universal joints of the other embodiments.Description of these test results will be omitted here.

[0254] In FIGS. 18(A) and 18(B), the support rings 32 are fitted to theouter peripheries 22 a of the trunnions 22. These support rings 32 androllers 34 are assembled (unitized) via the plurality of needle rollers36 to constitute the roller assemblies A capable of relative rotationstherebetween. More specifically, as magnified in FIG. 26, a plurality ofneedle rollers 36 are interposed between inner and outer racewaysurfaces, with the cylindrical outer periphery of a support ring 32 andthe cylindrical inner periphery of a roller 34 as the inner and outerraceway surfaces, respectively. Then, engaging means are arranged onboth axial sides of each roller assembly A so as to restrain axialrelative movements of the support ring 32, the roller 34, and the needlerollers 36. In the present embodiment, the engaging means on both sidesconsist of the engaging rings 33 and 35, which are fitted tocircumferential grooves 34 c and 34 d formed in the bore ends of theroller 34, respectively. The engaging rings 33 and 35 are set to therange of 0.5 mm≦W≦1.2 mm in width W, and limited to the range of HRC47-57 in surface hardness. This makes it possible to enhance the fatiguestrength against axial loads from the support ring 32 and the needlerollers 36, and improve the fatigue life of the contact surfaces withthe support ring 32 and the needle rollers 36. To fit the engaging rings33 and 35 to the circumferential grooves 34 c and 34 d, the engagingrings 33 and 35 are inserted into the inner periphery ends of the roller34 as elastically contracted in diameter, and then pushed to thepositions where the circumferential grooves 34 c and 34 d are formed.Then, as they reach the positions where the circumferential grooves 34 cand 34 d are formed, the engaging rings 33 and 35 elastically expandback to fit into the circumferential grooves 34 c and 34 d. The engagingrings 33 and 35 thus attached to the roller 34 make contact with the endfaces of the support ring 32 and the end faces of the needle rollers 36,thereby restraining these members from axial relative movements withrespect to the roller 34. Here, an example of the engaging rings 33 and35 is a split ring partially split by a slit.

[0255] In the constitution described above, the aforementioned variousmaterial improvements and surface modifications can be applied to theengaging rings 33 and 35 for yet enhanced fatigue strength against theaxial loads from the support ring 34 and the needle rollers 36 and forfurther improved fatigue life of the contact surfaces with the supportring 32 and the needle rollers 36. Moreover, these effects can beenhanced further by mounting the engaging rings 33 and 35 to thecircumferential grooves 34 c and 34 d in the roller 34 without play. Inthe present embodiment, the engaging rings 33, 35 are fitted to thebottoms of the circumferential grooves 34 c, 34 d with some interferenceso as to eliminate radial play between the engaging rings 33, 35 and theroller 34.

[0256]FIGS. 27-33 show other constitution examples of the rollerassembly A.

[0257] In an embodiment shown in FIG. 27, one of the engaging means inthe roller assembly A is the engaging ring 33, and the other consists ofa engaging collar 34 e. The engaging ring 33 is attached by fitting to acircumferential groove 34 c formed in one of the bore ends of the roller34. The engaging collar 34 e is arranged integrally on the other end ofthe roller 34. The engaging ring 33 can be fitted to the bottom of thecircumferential groove 34 c with some interference so as to eliminateradial play with the roller 34. The engaging collar 34 e is free fromany axial play and radial play with the roller 34 since it is integrallyformed on the roller 34. As compared with the embodiment shown in FIG.26, there is an advantage that assembling tolerance due to theengaging-ring constitution of the other engaging means can be eliminatedto reduce the axial clearances from the support ring 32 and the needlerollers 36 by half. Incidentally, the engaging collar 34 e may be formedon either end of the roller 34, facing to the trunnion bottom or thetrunnion extremity, while the engaging collar 34 e in this embodiment isarranged on the trunnion-bottom-side end of the roller 34. In otherrespects including the width W and the surface hardness, the presentembodiment is in conformity to the embodiment shown in FIG. 26.

[0258] In an embodiment shown in FIGS. 28(A) and 28(B), the engagingmeans on both axial sides of the roller assembly A consist of engagingrings 33 and 35 as in the embodiment shown in FIG. 26. Nevertheless, theengaging rings 33 and 35 in the present embodiment are provided withsteps 33 a and 35 a that are tapered to expand in diameter outwardly(with a taper angle β) so that the steps 33 a and 35 a are fitted to thebore ends of the roller 34 with interference. This can eliminate radialplay between the engaging rings 33, 35 and the roller 43. In addition,the contact portions S′ between the steps 33 a, 35 a and the bore endsof the roller 43 can receive the axial loads from the support ring 32and the needle rollers 36, thereby avoiding the torsional fatigue of theengaging rings 33 and 35 effectively. Incidentally, the rims of theengaging rings 33, 35 and the bottoms of the circumferential grooves 34c, 34 d have a slight radial clearances therebetween. An example of theengaging rings 33 and 35 is a split rings partially split by a slit. Inother respects including the width W and the surface hardness, thepresent embodiment is in conformity to the embodiment shown in FIG. 26.

[0259] In an embodiment shown in FIGS. 29(A) and 29(B), the outerperipheries of the engaging rings 33, 35 and side walls of thecircumferential grooves 34 c, 34 d are provided with tapered surfaces 33b, 35 b, 34 c 1, and 34 d 1 are arranged on respectively. The taperedsurfaces 33 b, 35 b of the engaging rings 33, 35 are taper-fitted to thetapered surfaces 34 c 1, 34 d 1 in the circumferential grooves 34 c, 34d. This can eliminate radial play and axial play between the engagingrings 33, 35 and the roller 34. While the engaging rings 33, 35 mayconsist of split rings, solid support rings as shown in FIG. 30 are alsoapplicable. More specifically, an annular portion 33 c (35 c) of theengaging ring 33 (35) is formed so as to slant in a natural state. Theengaging ring 33 (35) is inserted to the position where thecircumferential groove 34 c (34 d) is formed, and then an axial force Pis applied to elastically deform and erect the annular portion 33 c (35c). The annular portion 33 c (35 c) thus expands in outside diameter tofit into the circumferential groove 34 c (34 d), whereby the engagingring 33 (35) is fitted and fixed to the circumferential groove 34 c (34d) in the roller 34. In other respects including the width W and thesurface hardness, the present embodiment is in conformity to theembodiment shown in FIG. 26.

[0260] In an embodiment shown in FIG. 31, the engaging means on bothsides of the roller assembly A consist of engaging rings 33′ and 35′,and these engaging rings 33′ and 35′ are fitted to circumferentialgrooves 32 d and 32 e formed in the outer peripheral ends of the supportring 32, respectively. To fit the engaging rings 33′ and 35′ to thecircumferential grooves 32 d and 32 e, the engaging rings 33′ and 35′are mounted to the outer periphery ends of the support ring 32 aselastically expanded in diameter, and then pushed to the positions wherethe circumferential grooves 32 d and 32 e are formed. Then, as theyreach the positions where the circumferential grooves 32 d and 32 e areformed, the engaging rings 33′ and 35′ elastically contract back to fitinto the circumferential grooves 32 d and 32 e. The engaging rings 33′and 35′ thus attached to the support ring 32 make contact with the endfaces of the roller 34 and the end faces of the needle rollers 36,thereby restraining these members from axial relative movements withrespect to the support ring 32. In the present embodiment, the inneredges of the engaging rings 33′ and 35′ are fitted to the bottoms of thecircumferential grooves 32 d and 32 e with some interference so as toeliminate radial play between the engaging rings 33′, 35′ and thesupport ring 32. Here, an example of the engaging rings 33′ and 35′ is asplit ring partially split by a slit. In other respects including thewidth W and the surface hardness, the present embodiment is inconformity to the embodiment shown in FIG. 26.

[0261] In an embodiment shown in FIG. 32, one of the engaging means inthe roller assembly A is the engaging ring 33′, and the other consistsof a engaging collar 32 f. The engaging ring 33′ is fitted to thecircumferential groove 32 d formed in one of the outer peripheral endsof the support ring 32. The engaging collar 32 f is arranged integrallyon the other end of the support ring 32. The engaging ring 33′ can befitted, for example, to the bottom of the circumferential groove 32 dwith some interference so as to eliminate radial play with the supportring 32. The engaging collar 32 f is free from any axial play and radialplay with the support ring 32 since it is integrally formed on thesupport ring 32. As compared with the embodiment shown in FIG. 31, thereis an advantage that assembling tolerance due to the engaging-ringconstitution of the other engaging means can be eliminated to reduce theaxial clearances from the roller 34 and the needle rollers 36 by half.Incidentally, the engaging collar 32 f may be formed on either end ofthe support ring 32, facing to the trunnion bottom or the trunnionextremity, while the engaging collar 32 f in this embodiment is arrangedon the trunnion-bottom-side end of the roller 34. In other respectsincluding the width W and the surface hardness, the present embodimentis in conformity to the embodiment shown in FIG. 26.

[0262] In an embodiment shown in FIG. 33, one of the engaging means inthe roller assembly A is composed of the engaging ring 33 and a engagingcollar 32 g, and the other consists of the engaging collar 34 e. Theengaging ring 33 is fitted to the circumferential groove 34 c formed inone of the bore ends of the roller 34. The engaging collar 32 g isformed integrally on one end of the support ring 32. The engaging collar34 e is arranged integrally on the other end of the roller 34. Theengaging ring 33 can be fitted to, for example, the bottom of thecircumferential groove 34 c with some interference so as to eliminateradial play with the roller 34. The engaging collar 32 g is free fromany axial play and radial play with the support ring 32 since it isintegrally formed on the support ring 32. Besides, the engaging collar34 e is free from any axial play and radial play with the roller 34since it is integrally formed on the roller 34. In other respectsincluding the width W and the surface hardness, the present embodimentis in conformity to the embodiment shown in FIG. 26.

[0263] In the embodiments described above, the needle rollers 36 canadopt a variety of end-face configurations shown in FIGS. 34(A) through34(D). FIG. 34(A) shows a needle roller 36 with semi-spherical end faceshaving a radius of curvature of R′1. FIG. 34(B) shows a needle roller 36with partial spherical end faces having a radius of curvature of R′2.FIG. 34(C) shows a needle roller 36 with flat end faces having chamfersof along the corners. FIG. 34(D) shows a needle roller 36 with end facesof composite spherical configuration, having radii of curvature of R′3and r′ (R′3>r′).

[0264] The above-described various constitutions concerning the rollerassemblies are also applicable to the constant velocity universal jointsof the embodiments shown in FIGS. 20(A)-23, and the constant velocityuniversal joints of the embodiments shown in FIGS. 1(A)-17.

[0265] The following tests were conducted to confirm the effect ofsetting the width W of the engaging rings to the predetermined range andthe effect of limiting the surface hardness to the predetermined range.

[0266] [Tests on the Setting of Width W]

[0267] Tests were made on the constitutions shown in FIGS. 18(A) through26 with engaging rings set to below 0.5 mm, 0.8 mm, 1.0 mm, and above1.2 mm in width W, under the test conditions listed below. Then,evaluations were made for the fatigue strength against axial loads,to-roller mountability, and workability. The results are shown in Table13. In the evaluation fields, ⊚ represents satisfaction of the intendedproperty, and Δ dissatisfaction thereof.

[0268] (Test Condition)

[0269] torque: 686 Nm, revolutions: 250 rpm, operating angle θ: 10degrees

[0270] test time: 300 h

[0271] surface hardness of engaging rings: HRC 50 TABLE 13 WIDTH W (mm)UNDER 0.5  0.8  1.0  OVER 1.2 FATIGUE STRENGTH Δ ◯ ◯ ◯ AGAINST AXIALLOAD MOUNTABILITY ◯ ◯ ◯ Δ WORKABILITY ◯ ◯ ◯ Δ

[0272] It is confirmed from the test results shown in Table 13 that thesetting of the engaging ring width W to the range of 0.5 mm≦W≦1.2 mm canprovide satisfactory results in all respects, i.e., in the fatiguestrength against axial loads, to-roller mountability, and workability.

[0273] [Tests on the Limitation of the Surface Hardness]

[0274] Tests were made on the constitutions shown in FIGS. 18(A) through26 with engaging rings limited to below HRC 43, HRC 47, HRC 50, andabove HRC 53 in surface hardness, under the test conditions listedbelow. Then, evaluations were made for the fatigue strength againstaxial loads and the fatigue life on contact surfaces. The results areshown in Table 14. In the evaluation fields, ⊚ represents satisfactionof the intended property, and Δ dissatisfaction thereof.

[0275] (Test Condition)

[0276] torque: 686 Nm, revolutions: 250 rpm, operating angle θ: 10degrees

[0277] test time: 300 h

[0278] width W of engaging rings: 0.8 mm TABLE 14 SURFACE HARDNESS (HRC)UNDER 43 47 50 OVER 53 FATIGUE STRENGTH ◯ ◯ ◯ Δ AGAINST AXIAL LOADFATIGUE LIFE OF Δ ◯ ◯ ◯ CONTACT SURFACE

[0279] It is confirmed from the test results shown in Table 14 that thelimitation of the engaging rings to the range of HRC 43-53 can providesatisfactory results in both respects, i.e., in the fatigue strengthagainst axial loads and the fatigue life on contact surfaces.

[0280] While there has been described what are at present considered tobe preferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A constant velocity universal joint comprising:an outer joint member having an inner periphery provided with threeaxial track grooves, axial roller guideways being arranged on both sidesof each of the track grooves; a tripod member having threeradially-projecting trunnions; and a roller assembly mounted on each ofthe trunnions of the tripod member, the roller assembly being capable oftilting movement with respect to the trunnion and having a roller to beguided along the roller guideways in directions parallel to the axis ofthe outer joint member, wherein at least one component part of the jointis limited to a predetermined range in softening resistancecharacteristic value (R), wherein the roller assembly includes theroller to be guided by the roller guideway, and a support ring mountedon the outer periphery of the trunnion so as to support the rollerrotatably; the trunnion has a convex-spherical outer periphery; and thesupport ring has a cylindrical or conical inner periphery.
 2. Theconstant velocity universal joint according to claim 1, wherein thecomponent part is made of steel having a carbon content of 0.15-0.40% byweight, has a surface layer formed by carburizing and tempering beneatha predetermined surface, and has the softening resistance characteristicvalue R falling within the range of 705<R≦820 in Vickers hardness (Hv).3. The constant velocity universal joint according to claim 1, whereinthe component part is made of steel having a carbon content of0.15-0.40% by weight, has a surface layer formed by carbonitriding andtempering beneath a predetermined surface, and has the softeningresistance characteristic value R falling within the range of 705<R≦820in Vickers hardness (Hv).
 4. The constant velocity universal jointaccording to claim 1, wherein the component part is made of steel havinga carbon content of 0.45-0.60% by weight, has a surface layer formed byinduction hardening and tempering beneath a predetermined surface, andhas the softening resistance characteristic value R falling within therange of 630<R≦820 in Vickers hardness (Hv).
 5. A constant velocityuniversal joint comprising: an outer joint member having an innerperiphery provided with three axial track grooves, axial rollerguideways being arranged on both sides of each of the track grooves; atripod member having three radially-projecting trunnions; and a rollerassembly mounted on each of the trunnions of the tripod member, theroller assembly being capable of tilting movement with respect to thetrunnion and having a roller to be guided along the roller guideway indirections parallel to the axis of the outer joint member, wherein atleast one component part of the joint has a surface layer containing astructure in which carbide is distributed into a martensite matrix,wherein the roller assembly includes the roller to be guided by theroller guideway, and a support ring mounted on the outer periphery ofthe trunnion so as to support the roller rotatably; the trunnion has aconvex-spherical outer periphery; and the support ring has a cylindricalor conical inner periphery.
 6. The constant velocity universal jointaccording to claim 5, wherein the carbide is spheroidized carbide. 7.The constant velocity universal joint according to claim 5, wherein thecomponent part is made of steel material having a carbon content of0.80% by weight or higher.
 8. The constant velocity universal jointaccording to claim 5, wherein: the component part is made of steelmaterial having a carbon content of 0.15-0.40% by weight; and thesurface layer is a carburized layer.
 9. The constant velocity universaljoint according to claim 5, wherein the component part falls within therange of HRC 60-68 in surface hardness at least on its contact surface.